For general LaTeX questions, you can't beat the UK TeX FAQ. There are a few pieces of LaTeX documentation, though, that I keep sending people to. More than just manuals for particular packages, these give general hints for good typography and clean use of what LaTeX has to offer.
Publication quality tables in LaTeX
The manual to the booktabs package, which corrects some typographical errors in the setup of the standard LaTeX tabular environment. The package is very simple but it's use should be required by any professional publication, and Simon Fear's essay is also required reading. He argues for a spare, consistent style without vertical rules, and shows a truly horrific example of the alternative pulled from the LaTeX manual.
Short Math Guide for LaTeX
Michael Downes of the AMS wrote this guide partly as a manual for the most frequently used amsmath features, and partly as a general guide to mathematical conventions in typesetting. Anyone using LaTeX for mathematics should have read it.
Using Imported Graphics in LaTeX and pdfLaTeX
Keith Reckdahl's manual covers ways to use and manipulate images generated outside LaTeX in your documents, but it does much more. About half the manual deals with the LaTeX figure environment: how to control and influence float placement, how to modify the formatting of captions, and a long list of typical special-case effects such as side-by-side figures, numbered subfigures, splitting a single figure over two pages, and so on.
The Comprehensive LaTeX Symbol List
Scott Pakin's list collects several thousand symbols, and provides some useful suggestions for various ways to create any that might still be missing.