Reddit reviews Trigonometry the Easy Way (Easy Way Series)

Another book of number puzzles I loved when I was a girl about her age were the Sideways Arithmetic books. They add and subtract words to get another word, and so you have to apply logic to figure out which number each letter represents for it to add up correctly. I learned algebra mostly from the book Algebra the Easy Way. It introduces principles of algebra by having characters in a fantasy story face problems that require them to develop the algebra techniques to solve, and it comes with exercises to practice. There are other books in the series, including one for trigonometry and one for calculus.