[SCHiM]

I use that first reference extensively.

But you have to understand that it's just a reference, it doesn't give you the complete picture. It just shows you the important stuff when you already know where to look.

I've written partial disassemblers/assemblers. And that site has been a huge help to me.

My 2 cents:

Start with being able to decode the mov instruction, with all the different possible memory encodings. Once you understand how you parse the memory/addressing scheme of x86 it's suddenly a whole lot easier. And I agree that writing an assembler to start is probably easier, to write a disassembler it has to be complete, but an assembler doesn't have to support all instructions to work.