The rather conservative Victorian clergyman who received the letters must have been a little shocked: there are details of a squalid affair with a serving girl, fruity remarks about foreigners and literary vitriol.

Then again, maybe not. The sender was, after all, Lord Byron. The superstar Romantic poet's reputation for witty excess is affirmed by the sexual revelations, jibes about the Portuguese ("few vices except lice and sodomy") and barbed comments about his rival Wordsworth ("Turdsworth").

Sotheby's is to auction the most important series of Byron letters to come to the market in more than 30 years, some of them unpublished. They were purchased by a future prime minister, the Earl of Rosebery, in 1885 and have remained with the family ever since.

The letters shed fascinating light on one of literature's most charismatic figures, a man accurately described by his lover Lady Caroline Lamb as "mad, bad and dangerous to know". The Rosebery letters – all sent to his close friend Francis Hodgson – do not disappoint.

Sotheby's specialist Gabriel Heaton said: "Byron clearly enjoyed writing slightly outrageous things to a clergyman, but you do also get a very strong sense of the depth of friendship they had. There's a real intimacy."

About 15% of the letters' content is unpublished and unstudied. It includes references to Byron's affair with a serving girl, Susan Vaughan, that he ends when he hears she has been seeing someone else.

Heaton said: "Basically, he takes her as his mistress and he is never at any point saying he is going to be faithful to her but he expects her to be faithful to him and when he hears rumours that she isn't, she loses her job."

In another letter Byron talks about his time in Albania and Ali Pasha, the ruler, who impressed him as a "fine portly person with two hundred women and as many boys, many of the last I saw and very pretty creatures they were."

There is also much literary gossip and bitchiness. Byron and other leading poets of the time, such as Robert Southey and William Wordsworth, may have all been Romantics, but they were not friends. In the final letter he writes angrily about the denigration of a poet he much admired, Alexander Pope, by Southey and Wordsworth – or as he writes in the letter: "Southey and Turdsworth such renegado rascals."

• This article was amended on 8 October 2009. The original called Lord Rosebery a former prime minister - correct from the vantage point of the present but incorrect at the time he bought the Byron letters. This has been clarified.