Hi,
I faced similar problem and also searched for such book but couldn't get one in my city. I don't know about the plenty of such books available on web. Well, I would like to share my views in this matter.
The C used for programming a 8051 is only a subset of the original ANSI C. Not all of the features of C are supported. For e.g. the SPJ System's compiler supports pointers only upto one level. I don't know about other compilers. Secondly, the memory availability is scarce as compared to that available on a PC. while working on 8051 thru C u must take into consideration the amount of memory available and then allocate that for the variables. You can also define the variables at specific memory locations either internal or external. I think if u know the C and assembly both then it should not be difficult for u to learn the rules of the game.

Just an e.g.
assembly code: SETB P1.0
C code: P1.0 = 1; /*p1.0 should be
declared initially */

My advice is u go through some C Compiler's user manual. I have worked on SPJ System's (India) Compiler. The documentation is available online at www.spjsytems.com
You can even try www.keil.com, very much popular for its compiler and IDEs.

All the best.....