If you are already writing webpages than you should be familar with mark-up languages very similar to xml. Html is just another mark-up language. Most browsers understand both markup languages as well as others. Xml code is used by another program to basically "talk" to it in a language we understand to get it to do what you want it to. There are several good xml books out there but keep in mind that most of these deal with using mark-up language to manipulate web pages. The idea is the same though and most of the same rules and format apply to modding so its not a bad place to start. Just as the previous poster stated though, find out what programming languages you plan to learn and get resource books on those to start. Concentrate on one at a time and become fluent with it. Its just like learning a new language in the real world and once you learn your first the rest become easier as you go because of the similarities. If you learn latin you can begin to learn almost every other language out there. Same thing with programming. Xml is used for a lot of games out there but its not the only one and again its only the markup language used to speak to the machine. If you really want to program games then you will need to delve a little deeper and learn the actual machine's language. Good luck to you and btw its definately a rewarding career you are seeking. It can get frustrating at times but the end result is always worth it.
Sage
P.s. Here's a good link to learn a little bit to start. It gives you a bit of insight into what languages are used.
http://webferret.search.com/click?wf,+%2Bwhat+%2Bprogramming+%2Blanguage+%2Bis+%2Bused+%2Bto+%2Bprogram+%2Bg ames%3F,,www.euclideanspace.com%2Fsoftware%2Flanguage%2F,,altavista