I have several "must have" books or Bibles as some people call them (usually worth their weight in gold), here is a list of my currently used books:
I have a buch of Wrox books that I use to use all the time (especially ASP 2.0 and ASP 3.0), but for those topics I now use either MSDN library (I have the full install on my laptop) or Google.
What books would you not give up?