Complete Handbook/Finding information
Friends and colleagues
When you are searching for information, the best place to look for is amongst your friends and colleagues. They might not describe everything in full detail as you would expect from a book or technical document, but they are interactive, meaning that you can ask more questions as they come along. Another advantage is that they might reword their answers if you don't understand them.
Having friends and colleagues to ask questions to is a major advantage, especially if they also use the distribution you want to use. They might even give you on-site help: think of a private tutor. Make sure that this person doesn't mind you asking a lot of questions though.
When you are advanced in a certain topic, remember that other people helped you when you were still a novice and share your knowledge with other people. Be open for questions and help your friends and colleagues. Don't think you know all the answers though, even Einstein made mistakes.
When you can't find your answer amongst your friends or colleagues (or they aren't immediately available for help) your best bet would be to ask in User Groups. A Linux User Group (abbreviated to LUG) is a group of Linux users who gather to discuss Linux, give Linux-related presentations, etc.
User Groups are often a good place to start as well since they give you a friendly neighbourhood-like environment where you can ask questions, as simple as they might be, without being seen as a "dull newbie". A User Group is also a good place to find distributions so you can get the latest and greatest distribution for a small fee (most likely the costs of an empty CD/DVD) so you don't need to download it yourself.
In many User Groups you will often find events such as Install Fests. An install festival is a social event where you can bring your computer to and where other people will help you install your favorite distribution. Even better, they will help you tweak it, making it more performant, up to date and tailored to your needs.
Often called virtual user groups are the web site forums, places where you can find literally hundreds of people willing to help you in any way possible. On these forums (of which the Gentoo Forums are probably a perfect example) you can ask everything you want (as long as it remains on-topic).
Forums have a big advantage: you can always consult them, 24/7, and you will often find that they react quite fast. They also work as a great knowledge base where you can search through, hopefully finding someone who has posted your question before and has received all the information he needed. In that case, you don't need to re-ask (it is even considered rude to ask questions that have been answered not long ago).
Forums are also a great way to make friends: if you are very helpful yourself, you will undoubtedly get noticed. More than often will you find out that others live near you and share the same hobbies and interests. What better incentive do you need to get out to a pub and get a beer?
Books and guides
For specific subjects you might find that online guides prove to be a better resource. Such guides explain a single topic in great extend, often in a step-by-step construction, guiding you through the topic.
When you consult one of the more interactive resources (like forums) you will often be referred to an online guide which covers your subject. More than often those guides provide the best answer to your question, so don't be upset when the people don't answer your question but refer you to such a guide.
Gentoo has quite a lot of those helpful guides. If you think Gentoo is missing an interesting subject, don't hesitate to ask for one or even write one. Most documentation is written by volunteering contributors, so why not try contributing?
You will also find such guides, often in the form of a how-to, at the Linux Documentation Project.
When you want to learn more about a broader subject (like Gentoo in general) or in more detail than any guide could offer, you might want to buy (or download) a full book instead. O'Reilly has several dozens of books available covering a lot of subjects. A book is probably the most ultimate help you can find for self-teaching, but mind you, books often get outdated and aren't replaced as fast as online guides.
Some books are available online. More than often they are grown from a small guide to a larger one, eventually changing their layout from a guide to a book. This has happened with the Gentoo Handbook and Gentoo Security Handbook. Once they were only a few pages long. Now they span over a hundred pages.
You can find online books at The Linux Documentation Project.
Massive collaboration guides
Unlike the books, who don't get much updates, and the guides who do get updates if the maintainer is active, there are special kinds of online information pages that do get a lot of updates: massive collaboration guides, often in the form of so-called wiki projects.
Pages like these can be updated by any user who wishes so, making it quite easy to quickly fix issues and expand the document. But this fast updating has one major setback: people can easily sneak in more errors in the guide, or provide you with a step-by-step trail that is against the spirit of the subject you are interested in.
Manual pages are documentation pages that cover a single command. A manual page is a reference document that explains all possible options you can give at a command. Unlike guides they do not provide you with a step-by-step explanation on the subject and are therefore not interesting for guided help. They are however very important once you know the tool but want to know it better.
When you are inside a Linux system, you can obtain the manual page for a specific command or subject by typing man <subject>. For instance, to get the manual page for the emerge command often used on Gentoo:
Almost every possible command has a manual page on your system. Read the man page article for further details.
Another commonly used format to display information is the GNU Info browser. Whereas the man pages are a single resource containing a quick and dirty overview of the command (and its options), the info pages are a more extensive resource, dividing information in chapters, sections, ... and allowing you to browse from one subject to another.
To view an info page for a command, type info <command>. You'll be greeted by the info browser where you can navigate up and down using your arrow keys (line by line) or PageUp and PageDown (screen by screen). When you encounter a link (visualized by a * in front of it and :: after) press Enter to go to the page.
Using the keys u (up), n (next) and p (previous) you can navigate through the documentation easily. To quit, press q.
Please read the Info article for more informations.
Lots of software tools add documentation to your Linux system. This documentation can be in the form of a PDF document, HTML pages or plain text. In most cases, this documentation is stored in /usr/share/doc/<software title>.
For instance, the bzip2 compression utility has a manual (in PDF format) stored inside /usr/share/doc/bzip2-* (the version might be different on your system).
Most tools have immediate help available when you run the tool with --help or -h as one of its arguments. Do not hope to find much information here: in most cases the help provided is just a short summary of the available options.
For instance, for the emerge command (which lists quite a lot of detailed information):