Bach: Music in the Castle of Heaven. By John Eliot Gardiner. Knopf; 629 pages; $35. Allen Lane; £30. Buy from Amazon.com, Amazon.co.uk

WHEN John Eliot Gardiner grew up on his family’s farm in Dorset, he met Johann Sebastian Bach on the stairs every day. By some remarkable chance, a refugee from Silesia had given the Gardiners a portrait (pictured) of the composer to keep safe during the second world war. Painted by Elias Gottlob Haussmann in 1748, a couple of years before Bach’s death, it was one of a tiny handful of authenticated pictures painted during the great man’s lifetime. The young John Eliot found it a bit scary, but he nevertheless developed a lifelong fascination with the composer. Now 70, Sir John, as he has since become, is presenting his reflections about the man and his music in a new book.

Billed as a portrait of Johann Sebastian Bach, it is inevitably also a portrait of John Eliot Gardiner, who became a famous conductor and one of the leading lights of the period-performance, or “early music”, movement that started in the 1970s. Having experienced much of Bach’s music from the inside as a performer and conductor, Sir John is better placed than most to convey what it would have been like for Bach himself to stand in front of his musicians, and what went on in the composer’s mind when he wrote the music.

More than anything else, he is captivated by Bach’s vocal works: the cantatas, motets, Passions and Masses. In 2000, the 250th anniversary of Bach’s death, he took his Monteverdi Choir and English Baroque Soloists on a “Bach Cantata Pilgrimage”. In the space of a single year they performed 198 of Bach’s 200 surviving sacred cantatas in churches all over Europe and some in America.

This book is not a biography in the conventional sense—of which there are plenty already, some of them excellent—but an attempt to uncover the man through his music. Surprisingly little is known about Bach’s personal life. He was acquainted with grief. Orphaned at the age of nine, he lost his first wife, Maria Barbara, after 13 years of marriage. Of the seven children he had with her, four died before him. His second wife, Anna Magdalena, bore him 13 more children, but only six survived into adulthood.

Professionally, too, Bach seems to have had a difficult life. Born into a well-established family of musicians, he found it relatively easy to get his first job. But career opportunities were limited and he had to choose between a court appointment and a post as a church organist and music master. Each had its drawbacks. For a while Bach shuttled between the two (and was jailed briefly when he tried to leave his job at the court in Weimar for a better one), but in 1723 he accepted the position of Cantor, in charge of music at the Lutheran church of St Thomas, known as the Thomaskirche, in Leipzig, where he remained until he died in 1750.

There were many frustrations. The pay was not great; the city did not spend enough to provide him with the first-rate singers and instrumentalists his intricate music required; he was expected to do a lot of teaching; the council breathed down his neck when he tried to introduce anything too adventurous; and there was a lot of musical politics.

But in one sense he was in the right place. His ultimate goal, as he explained to an earlier employer, was to compose “a well-regulated or orderly church music to the Glory of God”. As a man of deep Protestant faith and a great admirer of Luther, Bach got the chance to do a lot of what he wanted most: to write glorious church music. No sooner had he arrived at the Thomaskirche than he started on a bout of furious cantata-composing. For the best part of three years he came up with a new one—about 20 minutes’ worth of music—for the church service every Sunday. During that time he also produced full-length Passions for each Easter and wrote a variety of other music. It was an unsustainable creative burst but left a lasting legacy.

Sir John analyses many of these cantatas in scholarly detail. He also explains the makings of each of Bach’s great Passions and of the sublime Latin Mass in B minor. He shows just how much thought went into selecting the texts and how consistently high was the quality of the compositions. This was truly “new music” of the day, like no one else’s, extraordinarily complex and bold. It made heavy demands on both performers and listeners. Even reading about it requires concentration. You either have to know the music very well or listen to it as you go through the text to make sense of it. Sir John’s book is not Bach for beginners, but it is very rewarding.

So what about Bach the man? There may not be much point in trying to draw a direct line between the personal qualities of this opinionated, crabby and often contrary workaholic and the marvel of his music. His reply to inquires about the secret of his musical success was deliberately opaque: “I was obliged to be industrious; whoever is equally industrious will succeed equally well.”

Sir John arrives at better answers by closely scrutinising the work. He discovers a wealth of hitherto unseen invention and ingenuity. But in the end, he finds, it comes down to an act of faith. Other composers, among them Monteverdi, Beethoven and Mozart, have achieved greatness in various ways, “but it is Bach…who gives us the voice of God—in human form.”