Mentioned by most, still another vote for K&R "The C Programming Language" - for the sheer joy of it! It still puts a smile on my face recalling how much fun it made learning C. :-) Don't think I've read as measured and not-boring for another language since. Close to as much fun to read was maybe Steve Maguire's "Writing Solid Code". Not strictly about learning C, but the examples are all in C. (some people dislike it's K&R not ANSI C)