Please recommends books I can get, from amazon? I need to get really good at mental math and math by hand, because I'm taking an exam soon without a calculator.
One classic text is The Trachtenberg Speed System, which actually has an amazing history: Trachtenberg created the tools for the book (speed mathematics, including multiplication) while in a Nazi concentration camp, without the use of paper or a writing utensil.
p.s. Don't let the \$91 price on Amazon.com worry you; you can find many used copies for less than \$10.
If you're looking for books about rapid arithmetic calculations, then these two books might be what you're looking for. They are Speed Mathematics and Short-Cut Math, which will cost you less than $15 for the both of them from Amazon. I bought them for my little sister a few years ago, since she wanted to learn some mental math tricks, and she said that they were both pretty good.