When you start out with a Linux installation you will normally get quite a bit of information along, not just the installation pamphlet but also substantial online help and information files as well as HOWTO files. This gives you a good starting point but after a while you will find yourself interested in knowing more, updating your system and basically staying informed. For simplicity this kind of information is here divided into several types, the type you subscribe to, information you search for as well as a bit on getting more specific help efficiently.
Even if you don't get printed information of some kind with your Linux packages you will along with any self respecting distribution get a number of directories with documentation of some kind, ranging from the tersest README files for most software packages to the more in depth HOWTOs, of which this is one.
Have a look in the document archive where most packages store their main documentation and README files etc. Also you will here find the HOWTO archive of ready formatted HOWTOs and also the mini-HOWTO archive of plain text documents.
The kernel source is, of course, the ultimate documentation. In other words, use the source, Luke. It should also be pointed out that the kernel comes not only with source code which is even commented (well, partially at least) but also an informative documentation directory. If you are about to ask any questions about the kernel you should read this first, it will save you and many others a lot of time and possibly embarrassment.
The online documentation is excellent for browsing and searching but don't dismiss the printed version altogether; if you cannot even get the machine to boot, how are you going to be able to read that piece of information you need to get the system going again?