Yeah, I already finished reading (for sufficiently buzzing-through values of "reading") Randall Munroe's What If?
tl;dr version: Buy it. Read it. It's funny, educational, and enlightening.
Now to plow through my little yellow bookmarks of quibbles and other reactions....
Page 39: Why does the Technetium hat say "Te" on it?
Page 47: A Mole of Moles: I find myself humming.
Page 116: "...we don't seem to pass any rhinoviruses back and forth with animals...." Interesting! I distinctly recall that some sciencey book I read as a kid said that ferrets could catch human colds. Current references say this is not the case (though they can catch influenza)*. I just unlearned something!
Page 152, footnote, regarding being hit by a bus: The description of the "being hit by a car" process would not apply in the case of a bus, 'cause the bus has a big flat front resembling the ground, only vertical.
Page 154: A typesetting issue in the Tsiolkovsky Rocket Equation has given us a rectangular delta. (There's another funny-looking equation on page 176.)
Page 156: We reach the end of "Everybody Out" having been introduced to Project Orion, but with no mention of laser launchers. I'd be really interested in having these included. (Yeah, I could go off and do the work myself. And wear out my poor little brain.)
Page 164: First mention of CON should be INT.
Page 175: Neutrino steam? I'm having silly ideas.
Page 197: But, by gliding away from the base of the cliff, won't you miss the cotton-candy pit?
Page 241: Unit peeve! I hate it when people use kilograms as units of weight. The Imperial system is goofed up enough by casual misuse of units; can't we try to preserve the meanings of the MKS units?
Page 261: Sorry, but the definition of "sunset" given on pages 228~229 would seem to exclude eclipses.
Page 304: What, no index?
* This suggests a way of telling a cold from the flu. If you can give it to your ferret, it's flu.