.CHM is a compiled HTML format used by Microsoft Help software. It also happens to work out as a very nice, indexed, searchable ebook format.
Here an excellent site on the CHM format --->
Unofficial HTML Help Specification
I'd personally find the module very useful. I've been wanting to use something like CHM to publish some things that I am writing, in a format more readable that PDF, but still somewhat portable. I was actually just about to start messing around with libchm myself...