Is it the one by Arthur Benjamin? Ds9 loves this book, we have it at home.
There is a relationship with math and magic. Since he can't borrow certain books at the library at his new school now (because of rules, rules and rules - roll eyes), he brought home a Magic book.
I have not read the Mental Math book. I pretty much leave him on his own when it comes to reading. I don't think it has confused him at all. He is doing CTY - AL1 now.