[dfjorque]

A pretty interesting (if quite long) introductory reading about the subject:

http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.ht...

[Adrock]

If you don't have time to read the whole thing and just want to learn how to add reasonably, I recommend http://en.wikipedia.org/wiki/Kahan_summation_algorithm