I am a huge fan of free documentation, even if I like to read printed books as well. In my opinion, the printed book still has the best available user interface that the computer screen and hypertext can only emulate but not surpass.
But on the other hand, Internet and the different forms of social media have several advantages over the printed book. Among the most obvious advantages are the free availability (if you happen to be living in the developed world with cheap broadband connections), and the possibility to update the information easily and often. Furthermore, there are several licenses that allow reusing the free documentation in different forms. Hence it should not come as a surprise that there are many good sources for free Linux documentation on the Internet.
In addition to the Linux Documentation Project, you should take a look at some of the more interesting documentation sites published in wiki format. For example, the Gentoo Linux Wiki includes a useful article about lightweight install of the Gentoo Linux.
Arch Linux is another favorite distribution used by many enthusiasts who want to keep their old hardware in use. Even if you are not ready to install and use Arch, you should take a look at Arch Linux Wiki. The page dedicated to Lightweight Applications is certainly useful for anyone interested in building a system with lightweight software.
And of course, you should not forget Debian Wiki or Crux Wiki, LQWiki and many other good documentation sites using a wiki platform!