I asked the same question a few days ago. taco-man pointed me to a few things, but in the end, just dive into the UIbuilder. If you want to learn the actualy XML, my best advise is to just open up the XML files in notepad or IE, then get one of these guys/gals mods and compare the code of the 2. That is what I am currently doing. This way you can see what code is doing what. As long as you know how to "Code/think logically" you will get it. My schedule is weird also, I am a full-time student, part-time job, married and addicted to games, so I am slowly learning the XML tags and attributes for XML.
If you want to actually look at XML and learn the concept and everything behind it goto
http://www.w3schools.com/xml/default.asp
I used this site a year or 2 ago or so to get me set for a job I had coming up that summer. I had to learn like 12 or so different languages/scripts to get prepared for it. It helps a little.