The Soroban / Abacus Handbook
A Lego Abacus.
The Soroban / Abacus Handbook is a how-to guide for basic and advanced abacus techniques. The abacus was originally designed for addition and subtraction, but you can also do multiplication, division, square roots and negative numbers (although not painlessly).
The abacus is the complement of the slide rule; it's good at addition but not multiplication, whereas the slide rule is good at multiplication but not addition.
Some slide rules "solved" this problem by having an addiator, a mechanical abacus-like device, attached to the back (examples here and here.)
I don't have an abacus yet (there aren't too many on eBay) but maybe I'll get the Abacus Book & Abacus at Amazon. It seems like a good deal.
In the meantime, I'll have to rig up a two-digit beginner's abacus using some Cheerios and drinking straws. (Or donuts and pepperoni sticks, depending on how hungy I am at the time...)