@Udo: Thanks. But there's no documentation released by some developer?
I try to explain myself: in these days I started to study how to program my Atmel by using Atmel Studio (or C ide equivalent like Code::Blocks). Everything ok, but since I realised that the arduino libraries are setted in all details, in some cases I can use them by Wiring. That being said, the datasheet whith registers is slightly less than a thousand pages: comparing every settings in code with every page it can take forever. If someone has done this before (the Arduino developers), it can be useful if I can find something like "PWM are in phase corrected mode by default" rather than "the ninth bit in serial has been disabled" and so on for every settings.
I'm quite sure that there's something written (excluding comments in the code, lol) because all of this it's a huge work and I doubt that there's someone who makes this basing all on his memory -off by heart-.
I remind to you that this is only for my personal curiosity (and that should be like that for every one: we're all in a hobbistic/academic environment :D), related to the "1,8 Kb of code loaded" for blinking a led (instead of 400 bytes using normal C), just to know what are in a detailed way 
@Mark: you're almost right, but I still hope in something of a better docs
thexeno