anyone knows somewhere to download some E-Book to learn F#?
The WikiBooks section on F# is pretty useful. It takes you right from the basics to advanced topics such as multi-threading and lexing/parsing.
Don Syme's and Luke Hoban's MSDN blogs are also great resources on the subject. Their various series of posts essentially act as chapters to a book, and are explained pretty well in my experience.
I learned a lot from Manning's Functional Programming for the Real World. It's available as a PDF, but obviously costs money...
Not sure if you mean free, but if you are willing to pay, Apress have a pdf version of the definitive F# book, and if you buy the paper copy, you can also get the pdf at a much reduced rate.
If you're after paid e-books, there is:
Both from Apress, available as both printed books and e-books. There's also "Beginning F#" and "The Definitive Guide To F#" from the same publisher, however, these are not yet available.
If you're after a free resource to learn about F#, you could try:
F# at Microsoft Research
The original website for the F# language at Microsoft Research, and contains a lots of good information and code samples.
The Learn F# section of Microsoft's MSDN site allows you to download 3 free chapters of the afore mentioned "Expert F#" book, as well as containing many other resources (articles, videos etc.) for learning F#.
There's also the F# Wiki which also contains many good articles, especially for getting started with F#.
There's some nice code samples on the Flying Frog Consultancy's F# pages. On the same site is The F#.NET Journal which is a series of high-quality articles on F# which has got some good reviews (however, they're not free).
Finally, there's also hubFS. A sort-of Digg/DotNetKicks type site specifically for F#.