You can also get developer manuals for the x86-64 (AMD64) architecture, used by AMD Opteron and AMD Athlon64, from AMD for free at hxxp://sausmaps.amd.com/AMDeMA/www/AMD64_programmers_manual_order.jsp

Note that you must be in the USA or Canada to order these. If you are not in USA or Canada, you can only get the PDFs of these books.

These are useful not only for the new x86-64 instructions, but they also document AMD's 3DNow!, 3DNow!+, and MMX+ instruction sets (which are used by AMD's Athlon MP and XP processors).