Expert F# 2.0 (Amazon link) is the most comprehensive guide to the F# language, co-authored by Don Syme, the designer of the language; Adam Granicz, IntelliFactory CEO; and Antonio Cisternino.
Expert F# 2.0 is a revised, second edition of Expert F#, reworked to be fully up to date with F# 2.0 and .NET 4.0.