Try to get hold of this Indian book: Vedic Mathematics or Sixteen Simple Mathematical Formulae from the Vedas (https://www.amazon.com/gp/product/8120801636/).

I have also heard of a Trachtenberg Speed system (https://www.amazon.com/Trachtenberg-Speed-System-Basic-Mathe...).

If you find Speed Maths interesting, you'll probably want to check out Trachtenberg's "Speed System of Basic Mathematics" book:

http://www.amazon.com/Trachtenberg-Speed-System-Basic-Mathem...

I have an indian edition of this book - http://www.amazon.com/Trachtenberg-Speed-System-Basic-Mathem.... I highly recommend exploring this method - its quite easy and the rules are simple to remember.