The Keil site has a bibliography; see also the 'Books' link here.

There's the book, "THE FINAL WORD ON THE 8051" on the Keil CD.

You should read the uVision Getting Started guide, and work through the example projects in it.

The Keil manual really only addresses the 8051-specific details of C51 - so you should read that, especially the Appendix on Writing Optimum Code
As you're familiar with assembler, you should have no difficulty with the particular architectural details of the 8051 (which is what often confuses "pure" 'C' programmers!)