Articles or book(s) about the functions behind a scientific calculator

[sa-penguin]

Knuth is - comprehensive. But not always easy to follow.
There's a section in Volume 2, on factoring prime numbers [and Fermat, I think] that make my eyes glaze over every time i try to read it.

A book on calculator algorithms is a fine idea, but what level of math prowess do you expect of the target audience?