The website Five Books consists of interviews with experts about their five favourite books in their subject. I was recently interviewed about my favourite popular applied mathematics books.
After making my original choice of books (ones that I use in my research) I realized that it was pitched at too high a level for a general audience. Identifying my revised choice was not easy, as I wanted to select books that contain a significant applied mathematics content, yet most books at the popular level are focused on pure mathematics.
I thought it would be good to include a book about equations. There are several such books, including It Must be Beautiful: Great Equations of Modern Science (2003) and The Great Equations: Breakthroughs in Science from Pythagoras to Heisenberg (2009). I chose Ian Stewart’s Seventeen Equations that Changed the World (2012), which has a strong applied mathematics slant.
After we did the interview, I saw an interview with Ian Stewart in Chalkdust magazine (a mathematics magazine produced by students at UCL), in which he said that the Seventeen Equations book is his favourite of all the books he’s written!
If you are wondering how to write a popular mathematics book, Ian gives some of the secrets in his article “How to Write a General Interest Mathematics Book” in The Princeton Companion to Applied Mathematics.
The complete set of Five Books interviews about mathematics is here.
Nick Higham 
Ian Stewart’s “Seventeen Equations that Changed the World” has won the 2017 Euler Book Prize of Mathematical Association of America: http://www.maa.org/news/authors-of-year-s-best-books-in-mathematics-honored