In my opinion it really depends what you are going to use XML for. In reality you can "learn" XML in a few minutes buy reading almost any tutorial on the subject. There is a lot more to the ways you can use XML but I would learn what you NEED to as you go along. For example if you're going to use XML for a PHP CMS, then sure the Sitepoing book is for you. If you're just going to use it to store information from your site without PHP you might want to learn just XSLT and so forth. It really depends what you want to use XML for. Right now I am creating a site for a hosting company using only XML documents. I then am writing the "design" into a single PHP file. Everytime the host wants to redesign their site I simply create a new PHP file that parses the documents. That is just an example of a way you can use XML.