Would you have tackled it first while trying to build such a library? I'd have started with 6502 or m68k as well. x86 is pretty huge, take a look at the x86 ISA docs sometime.
(We're talking 3-5 PDFs at 500+ pages each iirc, but that includes x86-64. Even if you go back and look at say the Pentium 3 manuals they are still quite large.)