The OS's publisher doesn't have a monopoly on printed matter relating to your OS, although for some of the more exotic OSs, your choices might be a bit slim. For most OSs, though, you can find a great deal of help from third-party documentation. For all but the most casual use, taking advantage of at least some of these sources of information is a good idea. Particularly for the more popular OSs, the selection is wide enough that you shouldn't have much trouble finding material to suit your exact needs.
Your Friendly Local Book Store and Library
One excellent starting point for help on most OSs is in books. Buying a book is a particularly good idea if you've purchased a CD-only version of an open source OS like Linux ...