The recommended book in C is:

The C Programming Language, Brian Kernighan and Dennis Ritchie, 2nd edition, 
1988, Prentice-Hall, ISBN 0-13-110362-8 (paperback version) or 
0-13-110370-9 (hardback version) 


The above text is often a challenging book to understand, even thought it
is fairly cheap.  If you would prefer and easier but longer and more
expensive textbook, try one of the following (in no particular order):


Applied C:  An Introduction and More, Al Fischer, D. Eggert, S. Ross,
2001, McGraw-Hill, 0-07-021748-3

C All-in-one Desk References For Dummies, D. Gookin, 2004, Wiley, 
0-7645-7069-2 

A Book in C, 4th edition, A. Kelley, I. Pohl, 1998, Addison Wesley, 
0-201-18399-4