Last year, someone linked to a book called
Build Your Own Combat Robot. I Amazon'ed it to my house, and sure enough, it is a good resource. Goes through a lot of key concepts including motor curves, drivetrain considerations, gearing ratios, wheel selection and size calculations, etc. Although some sections are geared (ahaha... no pun intended) towards the battlebots crowd ("Weapon Design"), robot design is robot design. Theres enough in it to make it worth the 20 bucks.