The full TeXLive installation comes with the documentation of its packages, usually under the path
The full installation takes about 4Gb, and is easiest done via the
install-tl script that can be found here. More information how to install in this way can be found in this related question
In the documentation you can find various helpful LaTeX introductions, such as
firstlatexdoc which shows you to get your first document
lshort which gives you a very good introduction into various aspects
latex4wp which has a focus on word processor users
latexcheat a cheat sheet overview
l2tabu an overview of bad practices
latex-doc-ptr an overview where to find more information
latex2e-help-texinfo the (un-)official reference manual
comprehensive an overview of an abundance of symbols and commands
You can directly load the corresponding
pdf of the documentation by typing e.g.
texdoc lshort on the command line.
Furthermore, as these documentation documents include their
.tex sources, you can even search the documentation via
grep, or using the following script.
You can find more information concerning references and documentation on the TeX Users Group web site or their links to further documentation, where you can also find recommendations for books and where you can download numerous resources.
Also have a look at the related question for more information, where you can find numerous links to pdf-files and Ebooks available for download:
Another good resource is the TeX Catalogue, which I prefer in the form of a Topical Index. Here you can also find a list of packages that might be interesting for your purpose, eg. for Computer science. It might be sufficient to just download or even print this list, as you need only the package names to find them in the documentation tree of TeXLive.