Description:
http://en.wikipedia.org/wiki/Assembly_language
A sample instruction set:
http://sca.uwaterloo.ca/coldfire/coldfire.html
Other resources:
http://sca.uwaterloo.ca/coldfire/
I never really needed a book to learn assembly. You just need to learn the instruction sets to be able to program. I guess there are good books out there to write "good/efficient assembly code". I am sure they are valuable to the people that do assembly programming professionally. But, the use of "strict" assembly programming is going down by the day IMO. Even with embedded programming - where traditionally u expect assembly to dominate. I saw some statistics that say C has taken over more or less.
Tell us if you need any more help.