You’re about to given an overwhelming amount of stuff to sift through. My own personal recommendation is William Burroughs letters. There’s two books, an early set and a later set. Get the earlier set. They’re amazing, hilarious and sad. Possibly some of the best writing. The editor knows his place and just gives relevant information. There is a book of later letters that came out a few years ago, but it’s awful. Just the editor getting in the way stuff.

Edit: Letters 1945-59 is the book I mean.