The manual, The Not So Short Introduction to LaTeX, is freely available as a PDF in a dozen languages. It contains a lot of useful information.
I highly recommend The LaTeX Companion by Frank Mittelbach et al., second edition (Boston: Addison Wesley, 2006).
It covers everything you’re likely to need – and more. At the moment, it costs about £35 from online retailers.
It can sometimes be tricky to find what you’re looking for in the index, but if you look back at the table of contents, you can usually work out which section it might be under.