"What If?" by Randall Munroe. It's what-if.xkcd.com collected in a nicely printed volume, with most of the articles that were published before 2013-11-26. This book is well worth reading even for people who have been following the web articles every week.
For people who prefer physically printed material, this book comes with easily legible text and high resolution stick figures. Some alt-text that were attached to images are not printed, and there are minor typesetting errors on pages 154 and 176, but generally I find this book much more user friendly than its online original. I would have bought this book even if it had no new content.
Beyond the user friendly format, there is lots of of new content, and readers only have to get as far as the cover to see it -- it's more than just the back cover, the reverse side of the cover and the inner cover is even more fun. At least 1/3 of the book are detailed questions and answers that were previously not published, and there are also a lot of short but fun questions (I thought the "sounds smart/dumb vs. actually smart/dumb" graph was especially good). Most of my favorite questions made it to this volume, and the new questions are all worth the time.
Overall, this has been a great offline version of some excellent science and math.
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