Float Exposed is an online tool to explore (or teach) floating point representation. The author, Bartosz Ciechanowski, also wrote an article Exposing Floating Point on his blog.
The Perils of Real Numbers (Part I) (broken link) by Steve Lionel
What every computer scientist should know about floating-point arithmetic by D. Goldberg in ACM Computing Surveys 23, 5-48 (1991). (pdf)
The pitfalls of verifying floating-point computations by David Monniaux.
An strange point a thread on comp.lang.fortran about testing for equality of reals.