There is an Arduino board that plugs into the r.P. I would connect the robot and I/O to it using standard Arduino shields. Especially as the r.Pi hasn't enough I/O lines to control a [nontrivial] robot on its own.
Simon Monk also has a ton of [cheap] books on these sort of things. See especially the pi-Python book. It has the RaspiRobot build in there.
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=simon+monk