Personally I recommend learning XHTML. It enforces good habits as it's not as lenient. Some of the rules may seem unnecessary, but at least it teaches you that there are rules, and how to look up what they are, fix errors, and overall develops good standards-compliant practices.
EDIT: To really address the original question, though, you don't need anything extra to write XHTML. Just make sure you have the right DOCTYPE declared.
The presumption in:
Is HXTML a library of HTML? Do I include an 'XHTML' include file to use XHTML? I'm just wondering how I use it.
question, - is at least tentatively, logical - which happens to be untrue just the same! And it is so -because of a (deliberate) naming confusion added to it, for "marketing" purposes they used to say (huh marketing!) but they both failed.
xhtml is, to say the least, a "Discontinued" and a failed attempt to hijack; Take Over & commercialize the Web Content Creation practice, and make people who are capable of using any Plain-Text-Editor to create marvels (we can everyday see), -incapable of doing so without purchasing their (xhtml coding, JAVA addictive) products, debuggers and so.
The Live Script and HTML prevailed, as good over evil always will...
Well, "most of the time" than.