Nicolas Sebrecht wrote:
> Does someone knows a good general (and free) documentation where to
> start ?

For working with the internals, specifically driver-side stuff and the 
mechanisms you need to write drivers, this book is really good (and free):
http://lwn.net/Kernel/LDD3/

I'm also fond of this book which complements LDD3 nicely (it talks about 
the scheduler, memory management, and a lot of other non-driver stuff 
which LDD3 purposefully omits):
http://rlove.org/
It's not free though.

Daniel