The "LaTeX2e Reference Manual" contains a lot of useful information about various commands. Is there any larger resource or book containing similar shorts descriptions of commands, but that also includes macros and options provided by various contributed packages?
Maybe the LaTeX companion (amazon link) is an interesting book for you. I read the german version and it contains a lot of information including most of the common and useful packages.
Furthermore mostly all packages are well documented. Try typing
texdoc <package name> to get the documentation or just google it.
I also take the ctan-ann RSS feed to see new updates an I found some useful packages I didn’t know by seeing them updated.
Most introductory/intermediate-level books about LaTeX do not only describe the "vanilla" system, but also include short references to quasi-standard packages like
hyperref. Examples are Kottwitz, LaTeX Beginner's Guide and Kopka/Daly, Guide to LaTeX.
The most sweeping "vanilla plus packages" reference book is Mittelbach/Goossens, The LaTeX Companion. It contains (often substantial) descriptions of about 200 packages prominent in 2004. This is also the drawback -- "rising stars" of the post-2004 era like
microtype are missing.