I gave this an extra star because I am neither a fan of Bellow, nor a reader of letters, and if you are both, certainly you couldn't go wrong with this. (In fact, if you're either or both, you can assume five stars.)
But even without an initial interest in the subject or form, I found this fairly absorbing. Bellow was a smart, entertaining, and supremely engaged man, and what I found most remarkable was how effected I was as those qualities faded, at the dimming of the light toward the end of this life.
Not being a reader of letters, I was unprepared for how thoroughly this one-sided, elliptical approach to conveying a life puts you right next to the writer, puts you inside him, despite the fact that he seemed so often to be aware that literary history was reading over his shoulder. It's a very personal experience, and unlike an epistolary novel, he's not making any effort to make sure you have all the facts. What you get are his feelings and thoughts about everything, and it's almost as if the physical man--the shell, the envelope--has been taken away and all that's left is what Bellow would have thought of as some yiddish word meaning "soul".
It was interesting to watch what age and success did to him toward the middle of his life, how he became somebody with an unconscious investment in the establishment, in property and respect, while still thinking of himself as being an outsider and shaker-up of things.
And then, of course, comes age, which not unexpectedly is horrible and sad. Despite the fact that he produced a last novel in, I think, his eighties, as well as a daughter at 84, his mind turned more and more inward, to the past, to memories, to his own childhood. (At least as represented here; he might have been living a full intellectual life that went unrecorded, but I read about the man who was drawn by his letters.)
All the feuds among the mid-century intellectuals, who reviewed whom badly, who didn't deserve which award, while it all seemed at first petty, it's all ultimately revealed as vital, the product of people bumping up against each other and vigorously disagreeing. Then, after a time, it becomes about who fell down and did or didn't break a hip.
I'm still not a fan, of Bellow or letters, but reading his letters forced me to get to know him in a way I haven't experienced in biography or even really fiction; it was more like some science fiction empathy device, where experience and emotion is downloaded directly into your mind, as bits and pieces of a life flash by, too fast, leaving you in tears at the end, when you emerge and remember that it was somebody else's life, not yours.