The Gimble is a nifty tool. It's not the most versatile book holder (it won't work with larger books), but it's small, modestly priced, and cleverly designed and named. I like using it to hold books open while I'm typing out passages.
Why gimble? The OED definition of gimbal helps out:
A contrivance by means of which articles for use at sea (esp. the compass and the chronometer) are suspended so as to keep a horizontal position. It usually consists of a pair of rings moving on pivots in such a way as to have a free motion in two directions at right angles, so as to counteract the motion of the vessel.This reading tool might be understood as a metaphorical gimbal: it keeps the book open in a horizontal position and allow for easy page-turning in two directions.
Gimble Hands Free Book Holder (Barnes and Noble)(Thanks, Elaine!)
Gimble (That company called "if")