[Third rereading; more if you count having read it online for years before the book.]

This is not the sort of book I'd usually review, any more than I'd review gravity or living on a planet with a breathable atmosphere. Not that I'm saying xkcd is as fundamental or necessary to my life ... — all right, I guess that is what I'm saying, and I stand by it. xkcd has been a highlight of my weeks since 2008; Munroe is easily the sharpest, most insightful and on-key cartoonist I've ever read, and that includes Larson and Watterson. What If is a delightful collection of his non-xkcd work, laugh-out-loud funny even on third-plus rereading; nerdy and sweet and exciting and sensitive and timeless. (Except for "Common Cold", p.114 - that one is quite timely as I write this in December 2020).

Why review it now? Because in the years since I bought the book my circle of friends, Goodreads and IRL, has expanded to include nontechnical people who might not be familiar with Munroe's work. This is my subtle way to encourage them to take a look.