I'd vote for this: http://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Calculation/dp/0307338401/ref=sr_1_1?ie=UTF8&s=books&qid=1286369636&sr=8-1 http://ww

I'd vote for this: http://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Calculation/dp/0307338401/ref=sr_1_1?ie=UTF8&s=books&qid=1286369636&sr=8-1 http://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Cal... and this: http://en.wikibooks.org/wiki/Mental_Math http://en.wikibooks.org/wiki/Mental_Math