Well right now the latest standard is XHTML which is HTML in XML.

You can use http://www.w3.org/ for reference on specific tags and for learning http://w3schools.com is a good start. As far as books go, the oreilly ones are usually the best:
http://www.oreilly.com/catalog/html5/index.html
http://xml.oreilly.com/

Depends on your needs, but if you are jut planning on making websites then learn XHTML. If you plan on doing something a bit more complex then you can learn XML itself to generate your own XML file formats.