I always try to have all recommended books available in our library. Also take a look at my special section there where I collect books which should be present at all times.
Kernighan/Ritchie, Programming in C. The bible of c-programming from the inventors. A classic text. Very short- compare this to nowadays documentation bloat.
The C pocket reference. A short book covering the latest developments in C. In my library.
The C Puzzle Book. Alan R. Feuer. A very short and nice book full of examples with C.