Books, sites, guides about mental arithmetic by hand and tricks?

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.

The Wikipedia page gives a pretty good overview of the book, and there is a bit about Jakow Trachtenberg on Wikipedia as well.

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.

There is also "Secrets of Mental Math" by Arthur Benjamin.