tldp.org is the linux documentation project (not really ebooks, in that these do not exist in print). it has some good info on intermediate/advanced topics, but some for starters too. for instance:
Introduction to Linux - A Hands on Guide [Only registered and activated users can see links. ]
or perhaps
GNU/Linux Command-Line Tools Summary [Only registered and activated users can see links. ]
other good stuff too. you may prefer to just dive in and start poking about and consult the documentation as necessary.. i can imagine just sitting down and reading any of these sorts of primers would be quite boring