I frequently refer to my copy of 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.
Stephens has two similar books: Monsters!: Draw Your Own Mutants, Freaks & Creeps and Heroes!: Draw Your Own Superheroes, Gadget Geeks & Other Do-Gooders. I have all three.