Earlier in the year, Mr. Gates lavished praise on an earlier book of Mr. Munroe’s: “What If? Serious Scientific Answers to Absurd Hypothetical Questions.” “My guess is that you haven’t spent a whole lot of time wondering what would happen if you pitched a baseball at 90 percent of the speed of light,” he wrote. “I haven’t either. But that’s O.K., because Randall Munroe has figured it out and explained it really clearly in his book ‘What If?’”

Mr. Munroe first learned that Mr. Gates had read and enjoyed his book when a relative sent him a screen shot of a photograph showing Mr. Gates reading his book. “I stared at it for a while,” Mr. Munroe said in a phone interview. “It didn’t really register. I didn’t know what to do.”

Mr. Gates said in the interview that he tries to fill his reviews with bits of information he hopes people will consider, even if they don’t end up reading the book. “I read textbooks related to global health but they are pretty technical for a general audience, so I generally don’t review them,” he said. “I make an exception for things like ‘Sustainable Materials: With Both Eyes Open,’ where the authors’ conclusions are important, and they help clarify some important basic facts. I like to share what I learn from books like that because I know most people won’t read the whole thing but some will read an 800-word review of it.”

He also shares some unexpected titles. Of Allie Brosh’s memoir, “Hyperbole and a Half: Unfortunate Situations, Flawed Coping Mechanisms, Mayhem, and Other Things That Happened,” Mr. Gates wrote: “You will rip through it in three hours, tops. But you’ll wish it went on longer, because it’s funny and smart as hell. I must have interrupted Melinda a dozen times to read to her passages that made me laugh out loud.”

Of the novel “The Rosie Project,” by Graeme Simsion, Mr. Gates wrote: “I started it myself at 11 p.m. one Saturday and stayed up with it until 3 the next morning. Anyone who occasionally gets overly logical will identify with the hero, a genetics professor with Asperger’s syndrome who goes looking for a wife. (Melinda thought I would appreciate the parts where he’s a little too obsessed with optimizing his schedule. She was right.)”