Part of learning assembly language is learning the hardware and the instruction set.

To quote Erik:

Anything you need to know about the 'standard '51', and more, is in "the bible"

here are the links to "the bible"
Chapter 1
http://www.semiconductors.philips.com/acrobat/v...ARCH_1.pdf

chapter 2
http://www.semiconductors.philips.com/acrobat/v...UIDE_1.pdf

chapter 3
http://www.semiconductors.philips.com/acrobat/v...WARE_1.pdf

Don't forget the tutorials here.

Something else that helps is to study examples of assembler programming. For these visit vendors like Atmel and go through their apnotes, most include sample code.

Michael Karas here often gives samples of well written assembler.

Dennis