The best documentation at this point would be something that teaches the details about how ethernet works. Once that is known, then the features of ethtool as documented in its man page should better match up with known ethernet features. You will have to decide what you want to do with ethernet (e.g. why you think you need to use ethtool). That should be clearer after knowing some of the details about ethernet operation and capability. Start at
http://en.wikipedia.org/wiki/Ethernet and follow some of the interesting links there. Maybe some thorough documentation can be encountered. Or Google for several of the terms you pick up from there to find pages that have concentrations of that information.
