George Shriver's new translation of The Letters of Rosa Luxemburg is the most comprehensive collection of her correspondence yet to appear in English. It transports us directly into the private world of a woman who has never lost her inspirational power as an original thinker and courageous activist in first the Marxist Social Democratic party, and then the German revolutionary group, the Spartacist League. She suffered for her convictions; jail sentences in 1904 and 1906 were followed by three and a half years in prison for opposing the first world war. Her brutal death at the hands of the militaristic Volunteer Corps during the 1919 workers uprising in Berlin has contributed to her mystique: she is revered as the revolutionary who never compromised. This collection of her letters reveals that the woman behind the mythic figure was also a compassionate, teasing, witty human being.

Annelies Laschitza, one of the volume's editors, observes in her introduction that the revelation in 1956 of Stalin's purges, along with new waves of activism during the 60s and 70s, reawakened interest in Luxemburg. My generation of left-libertarians did indeed hail Luxemburg's defiance of Lenin's "night-watchman spirit". Against his emphasis on the centralised party, many of us were drawn to Luxemburg's conviction that workers' action brought new social and political understandings.

Luxemburg's criticism of Marxism as dogma and her stress on consciousness exerted an influence on the women's liberation movement which emerged in the late 60s and early 70s. When I was writing Woman's Consciousness, Man's World during 1971, I drew on her analysis in The Accumulation of Capital (1913) of capital's greedy quest for non-capitalist markets, adapting it as a metaphor for the commodification of sexual relations and the body.

The awkward truth, however, was that Luxemburg herself had never identified with the feminist movement of her day. Moreover, she maintained a semi-detached relationship with the socialist women whom her friend Clara Zetkin organised in the Marxist Social Democratic party in Germany. Though she would be profoundly moved when they came to meet her from prison in 1916, and when they filled her flat with precious luxuries such as tea bags, cocoa, flowers and fruitcake, Luxemburg always carefully avoided being categorised as a "woman". Her resistance was partly strategic; she was determined not to be sidelined within the party. But it was also bound up with her theoretical conviction that class struggle was the key to change, along with a strong aversion to being regarded as a victim.

This recoil was rooted in her own experience. Luxemburg was born into a Jewish family in Poland in 1871; her father was a timber merchant, her mother was descended from a distinguished line of rabbis and scholars. While the Luksenburgs observed Jewish holidays, they sought assimilation; difference was to be denied. Nevertheless antisemitism, endemic in daily life, was sometimes unleashed in terrifying pogroms. As a schoolgirl, the young Rosa could sense her apartness from her classmates, not only because she was a Jew, but because a childhood illness had left her lame. She dressed carefully to conceal her limp and focused intently on books and ideas.

An early inspiration was the 19th-century Romantic writer Adam Mickiewicz, who challenged the oppression of the Polish people and made common cause with Russians defying the tsar. During the 1880s, when Luxemburg was entering adolescence, Polish and Russian women were being imprisoned and executed for their part in the revolutionary underground. Some of these formidable, emancipated women had studied in Zurich. So she was following in illustrious footsteps when she went there in 1889 to begin a zoology degree.

Already involved in socialist politics, she was attracted by a handsome, red-headed Lithuanian – Leo Jogiches. He was a man of action, accustomed to underground secrecy and interested in exercising power. Luxemburg's letters to him reveal a young woman who is intellectually questing and emotionally demonstrative.

Steeped in the history of the French revolution, she was intrigued by the polarities personified by Robespierre the ascetic and Danton the sybarite. In her letters to Jogiches she encompasses both extremes. She is at once passionate, sensuous, politically dutiful, bantering and acutely perceptive. She showers him with endearments: he is her "golden one", her "precious". But she chafes against his withholding of emotion, fears the waning of his desire. Luxemburg knew she bewildered him with her contrary impulses for autonomy and commitment, telling him in July 1897 that she felt "as touchy and skittish as a hare". When his response was to detach himself, she announced he was making her heart shrink. When he left, she cried.

Their political connection proved to be historically fateful. Jogiches operated easily within the Polish Marxist movement, but recognised that any influence he might have within the upper echelons of the German Social Democrats depended on the brilliant and personable Luxemburg. Braving Berlin in 1898, Luxemburg duly sent Jogiches informative reports on her political contacts with the leading left theorist Karl Kautsky and her agitation among Polish-speaking miners who were on strike in Upper Silesia.

At times she would rail against her tyrannical, patronising mentor, but she also depended on him. She fought him mischievously by writing cameos of the appreciative gallantry she evoked in the men she met in Berlin. And then, on 6 March 1899, she wrote to imagine them living together and buying a bookcase with a glass front. She was 29 and wondering whether she would ever have a child. In letter after letter she struggled to balance engagement in external action with inward reflection.

Luxemburg did not defy the conventions of gender openly, she simply circumnavigated them when it suited her to do so. Her relationship with Jogiches remained ostensibly secret; he was regarded politely by the Social Democrats as her "friend". It was a startling development when, in 1907, she fell in love with Zetkin's son Kostia. Luxemburg had known him since his teens and his mother had sent him to be her lodger. He was in his early 20s when they became lovers. "The sight of you gives me such aesthetic pleasure," she wrote in September 1908, reassuring him that for her the physical and the spiritual were intertwined.

Luxemburg's role as an international revolutionary figure took her to places from which women of her class were usually barred. Sitting alone in the Three Nuns in Whitechapel at the start of the Russian Social Democratic Labour party congress in May 1907, she snatches a moment to write to Zetkin. She relates how two women opposite her are joined by a constant stream of men. Noting that the men fail to remove their hats, she concludes they are up to no good. She hears raucous applause from an adjoining room And then she declares: "Some gypsy blood has been awakened" and she feels "an indistinct . . . desire to plunge into this whirlpool".

Yet Luxemburg was emphatically not a "new woman". When the Dutch socialist Henriette Roland Holst fell off her bicycle in 1904, Luxemburg wrote from prison to commiserate, joking about her own old-fashioned abhorrence of women on bikes. In June 1917 Luxemburg, stoical in the face of extreme deprivation, told her long-standing admirer, Hans Diefenbach, of her "helplessness in 'earthly matters'". When it came to train tickets, time tables and luggage, she was delighted to dump autonomy.

Humour bubbles up in the letters. Imprisoned in Wronke in January 1917, she reminds Luise Kautsky of the good times they had had "chitchatting and laughing" together. One night, coming from the home of the Social Democratic leader, August Bebel, they had walked through the streets in the night, singing away. She might have lambasted Luise's husband Karl, but this did not stop her maintaining a close, subversive friendship with his wife.

She had a sharp eye for incongruity. Announcing to Zetkin on 2 April 1911 that Lenin had visited for the fourth time, she declared: "I enjoy talking with him, he's clever and well educated, and has such an ugly mug, the kind I like to look at." Lenin, the consummate politician, paid court to Luxemburg's beloved cat, Mimi, declaring that "only in Siberia had he seen such a magnificent creature, that she was a barskii kot – a majestic cat". Mimi had "rolled on her back and behaved enticingly towards him, but when he tried to approach her she whacked him with a paw and snarled like a tiger".

Luxemburg's endless curiosity makes for graphic letter writing. Writing to Luise Kautsky from Italy in May 1909, she wonders why Genoese men get shaved at night "covered in white barber's cloths, with their noses tilted philosophically upward". She puzzles over the witticisms, too colloquial for her to understand, delivered by the heavy-set young postman "in his white shoes and Garibaldi hat". An admirer of good journalism, she grumbles about the flat jargon in Social Democratic newspapers, yet worries that she is not "a real writer" because she has never found writing easy.

She found it so hard because, as she explains to Henrietta Roland Holst in 1904, she wanted to convey "the living spirit of the movement". The dynamism was not just a matter of form – it imbued her thinking. Ideas take shape from within specific contexts and span out as she writes. This makes it difficult to pigeonhole Luxemburg. The Communist party would retrospectively label her as an advocate of a naive spontaneity. But while she saw action as generating a transformed consciousness, her letters testify to her belief in the need for revolutionary organisation too.

Ironically, the woman who hated splits was constantly embattled. Dangerously isolated, she went on fighting. As the years passed, it came to seem to others as if she had been somehow marked by destiny. Characteristically she accepted this with the minimum of pomp, remarking laconically to Luise Kautsky from jail in April 1917 that she was "'on leave' from World History". Prison allowed for reflection and, as the revolution in Russia erupted, individuals no longer seemed so important.

Luxemburg wrestled with a dilemma that troubled many of her contemporaries on the left and still resonates today: how to validate human beings' ability to change capitalist society, while giving weight to the force of historical circumstances. Her letters reveal a taut oscillation. Proletarian internationalism was being routed by war, yet she wrote on 11 February 1915: "Ça ira – it will go on."

Her death in 1919 was not to be the end of the story. In 1922, her former lover, Paul Levi, published a pamphlet she had written criticising the suppression of democracy in the Bolshevik revolution. It was akin to heresy and, under Stalin, her letters, archived in Moscow, were jealously controlled. Meanwhile the Kautskys were carrying Luxemburg letters with the rest of their papers from one European country to another as the Nazis advanced. In Germany, Mathilde Jacob, Luxemburg's devoted friend and typist, ensured that her collection was sent to the United States; soon after, Jacob was taken to a concentration camp.

And so it did, and does "go on", albeit in fits and starts, and with one step forward and several backward. Coincidentally The Letters of Rosa Luxemburg reaches us at a time when the peoples of the Middle East are asserting their aspirations for political, economic and social emancipation with formidable courage. The "living spirit" Luxemburg nurtured so strenuously has, once again, taken to the streets.