What's crazy is that depending on how deep you want to go, a lot of the information is not available in documents published by Intel. Fortunately, if it matters for emulators it typically can be/has been reverse engineering.
It was a nice period in history when proper RISC was working well, when we could get stuff to run faster, but getting more transistors was expensive. Now we can't get it to run faster but we can get billions of transistors, making stuff more complicated being the way to more performance.
I wonder if we ever again will get a time where simplification is a valid option...
http://www.rcollins.org/secrets/ http://www.rcollins.org/ddj/ddj.html
Notably things like undocumented opcodes, processor modes, undocumented registers, undocumented bits within registers, etc. It's not uncommon to release a specific set of documentation to trusted partners only. Back in the day intel called these "gold books".