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A good "gentle introduction" book is the Love book (440 pages): http://www.amazon.com/Linux-Kernel-Development-Robert-Love/d...

Isn't Bovet & Cesati a bit out of date now? I used to have a copy, but it was published in 2005, which means it was probably written in 2004.

Also, it's not kernel-specific, but this book covers a lot of system programming concepts (expensive though):