There was a great MITx course called Street-Fighting Math. Most of the techniques and ideas taught apply well to mental math. The course materials are freely re

There was a great MITx course called Street-Fighting Math. Most of the techniques and ideas taught apply well to mental math. The course materials are freely readable after logging into edX and enrolling. https://www.edx.org/course/street-fighting-math-mitx-6-sfmx https://www.edx.org/course/street-fighting-math-mitx-6-sfmx EDIT: Apparently the similarly-titled MIT Press book is freely available as PDF. Now that the course isn't currently running, I'd probably prefer the book. https://mitpress.mit.edu/books/street-fighting-mathematics https://mitpress.mit.edu/books/street-fighting-mathematics