Rubini's book "Writing Kernel Device Drivers"
http://www.kernel.org/pub/software/scm/git/docs/gittutorial.html - git tutorial
http://www.tldp.org/ - Assorted online guides and a mine of information.
The folowing are helpful for people inexperienced in working within a linux environment:
http://www.advancedlinuxprogramming.com/ - General linux programming
For guides to specific GNU/linux tools:
http://www.gnu.org/doc/other-free-books.html
http://www.gnu.org/manual/manual.html
Required reading for anyone planning on low level code:
http://www.xml.com/ldd/chapter/book/ - Linux Device Drivers
http://people.mech.kuleuven.ac.be/~bruyninc/rthowto/
http://www.fdn.fr/~brouchou/rtai/rtai-doc-prj/
Recommended reading for an undersatnding of the principles of communication within emc:
The RCS Handbook by Albus, Gazi, Moore, Passino, Proctor, & Shackleford.
http://www.ece.osu.edu/nist_rcs_lib/
http://www.isd.mel.nist.gov/projects/rcslib/index.html