Even if you haven't read any posts from the blog Hyperbole and a Half there's still a good chance you've seen a bit of Allie Brosh's work. The "all the things" meme inspired by this image:
came from her post This is Why I'll Never be an Adult. That post is as good an introduction to her work as any.
Much like the What If? book, Brosh has collected many old posts and combined them with new content to make a spiffy little book full of awesome. Again, like with What If?, if you've read the blog then there isn't necessarily enough extra in the book to warrant the purchase beyond perhaps a desire to bestow monetary recompense on a source of previously free joy. And that's a totally valid thing to do. Me, I picked up the book used for $5 as part of a charity sale so I guess I'm at least helping sales in the less quantifiable but still important secondary market. And also, charity! I'm not a monster.
Either via the book or the blog I do recommend reading her work. It's frequently silly and hilarious which makes the random moments of poignancy all the more powerful. Her two posts on depression (#1, #2) are maybe not the best places to start reading the blog, but combined form a pretty amazing look into a chronically misunderstood condition. Also the blog/book has lots of dog stories if you're into that sort of thing.