If you don’t know where is the documentation of a package included in your TeX distribution because, for example, you are not familiar with the Unix filesystem locations, then there is a simple way to get the pdf documentation related to any package.

When you get the name of a package, say for example classicthesis, then type Alt+F2 to get a prompt (it depends on your desktop environment, but if you use Gnome or KDE it will work, and if not, you can use gmrun). Then, enter “texdoc” followed by the name of the package. Here is a screenshot of what you get in Ubuntu after typing Alt+F2 :

After typing “texdoc” and the name of the package, you will get its related documentation in pdf format :

Here is a link toward a description of the texdoc command. Not really useful, just in case you want it.

If you don’t want to use texdoc, or just want to get the .tex format of the pdf documentation, then look at this directory if you have installed TeXLive in Ubuntu with the usual method :

/usr/share/doc/

If you use the default file manager of Ubuntu, type TeXLive, and you will have all the documentation about latex, xelatex, etc. If you use pdflatex to compile your files, then you might find all the doc you need in :

/usr/share/doc/texlive-latex/

Finally, if you don’t have installed TeXLive with the usual method, but with the script install-tl of the TeX User Group, then look here :

/usr/local/texlive/2010/texmf-dist/doc/

By the way, if you are interested in the Unix filesystem hierarchy of GNU/Linux, then here is a link you might find interesting :
http://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard

And here is for FreeBSD users :

http://www.freebsd.org/cgi/man.cgi?query=hier