I might be wrong, but I thin it's a bit too early for you to buy an assembler book. Try to search for good tutorials which explain also the background WHY you are patching something or WHY you should look at adress xxyyzz for a serial. If you found such a tutorial, try to find a nice Assembler reference on the net.

http://yanorel.pcupdate.easyspace.com/masm/ewtoc.html or

http://protools.anticrack.de/files/docs/asmhelp.zip

would be a good start. This will save you some money for now and still teaches you some stuff.

greetings
Mav