I've only found two books on C worth the money. One is K&R, the other one (which I haven't seen mentioned here) is "The Standard C Library" by P.J. Plauger. It not only presents the ISO standard for the library, but Plauger's commentary on the standard, how to implement the standard C library, along with his implementation. It explains some of the murkier aspects of the C library, along with how to go about writing portable code.