The spec I use this all the time, for obvious reasons.
Expert F# (I use it on Safari - this is the incomplete/sample Google Books entry)
The basic syntax of F# - classes, interfaces, and members Useful disection of an F# type declaration
F Sharp Programming - Wikibooks, collection of open-content textbooks Occasionally useful, but somewhat out of date. Better to turn to Expert F#.
hubFS forums Lots of useful information in the hubFS forums. (Think of this as the F# mailing lists; for whatever reason, the F# community seems to prefer forums to mailing lists.)
Sep 2008 release known issues