Info Vault and the "AboutSIDL.doc" and the "SIDL.xml" documents in the EQ>uifiles>default directory.
Even though the title says it's about graphics in particular,
this thread in particular I found helpful in explaining some of this stuff to me.
And I loved
SidlWidl for helping get me started as well. Once I was able to "see" what I was doing and then compare my "new" file to an old default one, and check out what the differences were in them ...
Plus, one of the threads had a
link to an actual XML tutorial, of which SIDL that EQ uses is only a very minor minor part.