A grammar book! I don't need much more than that, but it was a very humorous read about a "comma queen" from the editorial staff The New Yorker. I learned a few tidbits of grammar rules (my biggest takeaway I hadn't known before was about restrictive "that" vs nonrestrictive "which"), though many I had already known, being of course, a bit of a grammer nerd myself. I was surprised at the animosity towards calling it the "Oxford" comma (though do a Goolge search for Oxford, Walken, and Shatner commas; it's great).
The fun random trivia was how Noah Webster wanted to rename certain letters of the English alphabet. As if we don't have enough rhymers, he wanted to call h "hee", w "wee", and y "yee". Sounds silly, but think of Bee, Cee, Dee, Eee, Gee, Pee, Tee, Vee. His only success was changing "zed" to "zee." Kinda makes me want to call it Zed now, even if I hadn't already thought that was cooler anyway.
The other interesting trivia concerned my own local employer: Lands' End. Apparently I didn't even know where the apostrophe went, but the company did start out as the more appropriate "Land's End" but a too-expensive-to-replace printing error on their first catalog cemented the current name in history.