There is a very nice online book called "The F# Survival Guide", previously hosted at ctocorner.com. Unfortunately, for some reason that site is no longer available. Several F# community members have tried to work out why, but so far to no avail.
In any case, the book is HTML and the most recent version is available at the internet archive web.archive.org. So if you're looking to read it, here is a link to an archived edition of The F# Survival Guide.