Traditionally, the only sources of information on a program under UNIX were the source code, and the man pages. And not many people had the patience to read the source to find the answers to their questions. The problem with the man pages are far as new users are concerned is that they are not very intelligible. And this is because they were written by programmers for an audience of other programmers. Hardly the sort of thing needed by new users trying to figure things out.
The GNU (which stands for GNU's Not UNIX, an example of a recursive acronym) project introduced a new family of documentation, known as info. Another name for it might be Texinfo (or TeXinfo of TeXInfo). As many of the standard UNIX commands are now provided by the GNU project of the Free Software Foundation (FSF), many man pages bear a message saying that the man page is now obsolete. I'm sure this is very refreshing for the new user, not only can they not read the man page, it's not even up to date.
The last sources of information which most (nearly all) Linux system come with are the /usr/doc and /usr/share filesystems. The documentation in /usr/share is somewhat like the man pages in that it can be hard to find or understand. Also, most of the data in /usr/share is not meant to be read anyway.
There are professional type books from the Linux Documentation Project. There are HOWTO's and mini-HOWTO's, and there are Frequently Asked Question (FAQs) lists. And these often come in a variety of forms: text, SGML, LaTeX, HTML, PostScript, .... One recent RedHat 6.0 install which didn't look complete was 160 MB, and a nearly complete SuSE 5.3 install was over 600 MB. So, there is lots of information there to read. It's biggest problem is that it is not indexed well, or easily searchable.
There used to be a set of documentation accessed by the help program, but this looks to be deprecated.