Hi, you will be flooded with suggestions bcoz there r literally N no. of reading materials available on line as well as off line. i'd also like to suggest u something.

1) If u r totally new to the microcontroller world, read some basic book like this: written by Kenneth Ayala - 8051 micorcontroller - architecture and programming
2) if u've got a basic understanding of micros and particularly the 8051, then simply start writing small programs in assembly and try to simulate them in some simulator available on the web.
Intel's user manual for MCS-51 is also a good document but too big.

All the best........