Every programmer forum gets a steady stream of novice questions about numbers not 'adding up.' Apart from repetitive explanations, SOP is to link to a paper by David Goldberg which, while very thorough, is not very accessible for novices. To alleviate this, The Floating-Point Guide, is a floating-point equivalent to Joel Spolsky's excellent introduction to Unicode.
you can learn quite a few things about the intricacies of the IEEE 754 standard, and just how difficult it is to compare floating-point numbers using an epsilon.
stumbled across this, would be very useful.