Reddit reviews Math Magic: How to Master Everyday Math Problems, Revised Edition

Reading this book as a kid means I always calculate tips in my head for my advanced mathematics loving friends.

If you are really curious about those tricks, there are books with more. I can't recommand a specific one though since the book I had is in german. I also may point out that most of the tricks are for special cases (like squaring numbers that end on 5), so you would need to train lots of different tricks to get really good at it.

I personally started to calculate the total price I need to pay in the grocery store when buying stuff. That is not too hard and ensures that you'll train on useful numbers.

That's the way I'd do it but I read math magic when I was a kid and it taught lots of simplifying strategies that I didn't learn in school