Where can I find XT-IDE (8bit IDE, XTA) specifications?

From googling XTA I found the wikipedia page, which mentions XT-IDE.

The wikipedia page mentions the ST351 A/X as being a 40MB XTA drive.

Googling "ST351A/X xta programming model" led to a PDF (3rd hit, as of this writing) about the Amstrad PCW.

Why do you care about the Amstrad PCW? B/c it used a ST351 A/X in "XT mode", and said PDF contains references to where the registers lived in the XT: ports 0320h-0323h.

Now you still need to track down how ports 0x320 to 0x323 worked in an XT, but that's closer than you were.