This is Not a Math Book, by Anna Weltman
Patterns of the Universe: A Coloring Adventure in Math and Beauty, by Alex Bellos
Mathematical Mindsets, by Jo Boaler
Intentional Talk: How to Structure and Lead Productive Mathematical Discussions, by Elham Kazemi
Dan MacKinnon wrote a lovely review of a book I hadn't heard of before, at his blog, Math Recreation. Here's the beginning of it...
In The Puzzle Universe: A History of Mathematics* in 315 Puzzles (TPU), Ivan Moscovich stretches the concept of puzzles to encompass almost anything that combines curiosity and playfulness (playthinks is his preferred term for this more general category of puzzling items). No surprise - these playful curiosities are inherently mathematical. In an informal and accessible way, Moscovich details the development of these puzzles, revealing their surprising family resemblances and the deep mathematics behind their playful exterior. [read the rest at Dan's blog...]
And of course, there's my book, Playing with Math: Stories from Math Circles, Homeschoolers, and Passionate Teachers, along with all the other cool books at Natural Math.
*This link goes to bookfinder.com, which will point to other sites. It's the best way I know of to find the least expensive copy available. (My other links point to the sites that were cheapest at bookfinder on the day I wrote this.)