Buy the cheapest crapiest book out there for XML learning,XML it self is nothing more then a fancy text standard.(yes 21 days book should do)
The tricky part is when you learn to create/parse XML using SAX(Simple API for XML ) or DOM (Document Object Model) based API's.
Then the next part comes to action that which languages will you be wroking with.If java
Then here is a detailed tutorial on Java/XML.Its a pretuy good tutorial.
Unfortunately I have no idea for any other languages( I know there are parsers for C/C++ though).
If XML/Java I can probably help you a great deal.