He probably doesn't want a book, but I'm going to recommend he beg, borrow, or steal K&R (ANSI ed). It does a great job, and with his backround it's just the level he needs.
What is more interesting is all the lost voodoo, that everyone half decent was expected to just know, especially if you were doing anything embedded or kernel hacking. This will help him to understand why it's low level when needed.
Like the lost art of structure packing (that most of us used to do automatically without thought). No one does that any more outside of embedded systems.
http://www.catb.org/esr/structure-packing/
Or some of the evil low level pointer hackery, but I'm not turning up anything great on google about this. Most sites claiming to have are dull run of the mill pointer operations. No unions of structs of arrays of pointers to functions doing evil trickery with inline assembler in sight! Amateurs!
SO post on "hidden features of C" https://stackoverflow.com/questions/132241/hidden-features-o...
Not very hidden most of them, but hey.
Obfuscated C Contest is good for making you friends' head hurt. Some really clever ones, so worth a look.
http://www.ioccc.org/
I think I may have spent too long C hacking in my 20s... :)