If you're interested in utilising the GPIO port more fully, take a look at Raspberry Pi Measurement Electronic: hardware & software by Yury Magda. It's Kindle only (sadly) and can be seen at
You can view contents page and introduction (which gets as far as 'Expanding the GPIO Port with an I2C Interface') to get an idea of the book .
Looks like US$10.61 well spent.
Usual disclaimers.
(BTW, Raspberry Pi for Dummies is pretty good too)
Any other recommendations?