I picked up a fun full-color book on Saturday called Robots! Draw Your Own Androids, Cyborgs & Fighting Bots, by well known animation designer and comic book cartoonist Jay Stephens. It's only 64 pages, but it's packed with tips and examples of whimsical robots, with lots of examples of limbs, bodies, heads, control panels, sensors, etc.
I also learned that Stephens has two similar books for sale: Monsters!: Draw Your Own Mutants, Freaks & Creeps and Heroes!: Draw Your Own Superheroes, Gadget Geeks & Other Do-Gooders. They are a bargain at just $5.95 each. I just ordered both of them!