I certainly didn't expect to have to think about spoilers when reviewing a comedy book by a stand-up comedian, but it turns out that I have to assure you that the only spoiler in this post is the fact that I have to say this at all, and therefore, that there's things to be spoiled.
If you've watched Mike Birbiglia's comedy acts, you may expect this book to share a lot in common with them. There's only a few actual jokes that you've heard before in his acts that made it into the book, like about Mike's wheelhouse of skills (English muffin pizzas, and not jumping out of trees), part of how his brother gets the nickname Joey Bag Of Donuts, his parents having a porn virus, and a few more. But the vast majority of the book is material that wasn't familiar at least to me, having seen most of his specials.
The biggest commonality is just the tone Mike takes, the kind of humor, and the fact that it's very personal. He cranks up the personalness, in fact, which might seem surprising: his act is mostly him telling stories of his own life, so how could it be more personal? Two ways. First, the stories he's telling in the book are more personal things, and more so as the book progresses. In a way, this is an autobiography; it's a big coming-out about things in his life that he's never told us before. And second, the tone is much more intimate; he's making confession about things that were difficult, even painful or scary, for him.
You might think from that that it's going to be a bummer, but it's really, really not. At no point does he waver from being funny. It's not like how a comedy writer sometimes completely shifts gears to be emotional (like Dave Barry's extremely rare departures into writing about family tragedies). Not that that would be bad, but that's not what Mike does. There's no point where the emotional impact is opposed to, or competing with, the humor. They're entwined.
The book was a quick read, and some of that is because it's kind of light, but some is just because it's surprisingly engaging. You expect a stand-up comedian's book to be something you can dip into and back out of, so it was surprising to find myself gripped with suspense towards the end, eager to see what was coming.
Overall, as you'd expect for a book, the sheer density of laughs-per-minute is lower than a stand-up act. That's unavoidable, and Mike wisely doesn't try to avoid it. The result feels unaffected, uncontrived: it's Mike telling you about his life and being funny all the time doing it just because his thoughts are always funny even when they're also serious. It's really impressive when you think of the craftsmanship especially because while you're reading it you won't be thinking of the craftsmanship; you'll feel like even Mike wasn't thinking about it, either.