There are less and less reasons to bust out and start writing your own assembly language these days since the C/C++ optimizers almost always get it better. But just in case you'd like to try your hat at it both AMD and Intel make it easy for you to get processor architecture reference material.
AMD
Materials online: http://developer.amd.com/documentation.aspx
Order literature: http://www.amd.com/us-en/Processors/TechnicalResou...
Intel
Materials online: http://developer.intel.com/design/archives/process...
Order literature: http://www.intel.com/design/literature.htm
And if you're a new recruit to the world of assembly language you might want to take advantage of Randall Hyde's Art of Assembly Language Programming website: http://webster.cs.ucr.edu/