I don't think it an accident that I actually had to go through the intense experiences of rehearsal and performance of the late works before the fog lifted, as it were. I realize that most people don't have the same opportunities. Sessions, like Schoenberg and Webern, requires a great performer far more than even Stravinsky, whose textures are generally less opaque and who frequently presents you with an immediately-exciting surface. The value of books like this one, in my opinion, usually lies in providing a confirmation or a clearer view of a composer you admire. If the music doesn't interest you in the first place, reading the composer's correspondence won't usually awake that interest.
Few composers lead especially interesting lives.
Other than the wife-killer Gesualdo or the murder victim Leclair, the lives won't have you turning pages as quickly as the unauthorized Madonna bio. It's also the rare composer who pours his thoughts on art into letters, although he may very likely let fly with a nasty remark on a contemporary or, more rarely, a bit of praise. Letters usually get written to satisfy the demands of living or even getting a living. Discussions about art usually get written down because someone's willing to pay, although the Internet may have begun changing this, perhaps profoundly. Anyone who has heard a Sessions work should recognize at least a musician of uncompromising integrity.