Some resources for you:

Books:

Now that you know APPLE ASSEMBLY LANGUAGE: What can you do with it? by

http://www.1000bit.it/support/manuali/apple/AALby_JH_Gilder.pdf

Machine Language for Beginners by Richard Mansfield:

http://www.atariarchives.org/mlb/

The Second Book Of Machine Language by Richard Mansfield:

http://www.atariarchives.org/2bml/

Stuff:

Bob Sander-Cederlof's Apple Assembly Line archive:

http://homepage.mac.com/bobsc/aal/

John B. Matthews' notes on cross-assembly (developing on a fast machine, running on an Apple):

http://home.roadrunner.com/~jbmatthews/a2/cross.html