The letters in this book- written mostly to Allen Ginsberg with letters to other notable recipients like Kerouac and Gysin scattered among them- are amongst some of the most entertaining and well written by any writer that I've come across.
They cover a key period in Burroughs's life, commencing in the mid 40s when he was trying to make a go of being a family man and a farmer. Odd to think of the latter day scourge of authority and conservatism worrying about how much his carrot and cotton crops will fetch, but he did all the same. Then comes the move to Mexico, the fatal 'William Tell routine' gone wrong when he shot his wife Joan, and the years of addiction and wandering. All of this is fully documented here.
If you want to learn more about what made Burroughs the man and the writer he was, and how his later world view developed, I think a lot of the answers are in these letters. Certainly without the letters I don't think he could have been the writer he was. They were his lifeline- at one stage he comments on how much he needs an audience, and for a long time Ginsberg and Kerouac fulfilled this role- at a time when he was effectively serving his writing apprenticeship, looking for things to write and still without an audience. The letters effectively kept him going and gave him a chance to develop. Also, we see his world view change and mature and by the end of this book we've seen him come to terms with his status as an outsider.
From would-be farmer worrying about how much his crops will yield, to a fully-fledged avant-garde artist in 15 years is pretty good going, Along the way there's a lot of hardship, a lot of moaning about his lot and above all some genuinely funny passages. You can gain a lot from these letters in their own right, and if you've always been left cold by Burroughs or put off him, these letters will help you understand a lot more about why he wrote as he did and where his particular sardonic take on the world came from.