PDF Manual
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.
Published Guides
I highly recommend The LaTeX Companion by Frank Mittelbach et al., second edition (Boston: Addison Wesley, 2006).
Paperback details:
ISBN-10: 0201362996
ISBN-13: 978-0201362992
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.
LaTeX for Humans 
Hey, your link to the tutorial “Not so short introduction to latex” actually goes to “Begin Latex”, another tutorial. Which of them do you really mean?
Sorry about that – I meant “The Not So Short Introduction to LaTeX”.
You can find this at:
Click to access lshort.pdf
I have now amended the links to this document on my posts.
I wouldn’t recommend The LaTeX Companion to new users. Once you’ve got your feet wet, it’s very handy (although most everything in it can be found in the documentation on your system), but I think it would really confuse people who didn’t understand the basics.
What about Lamport’s book, which covers the basics? Or Gratzer’s More Math into LaTeX? I edited the third edition of Gratzer’s Math into LaTeX, so I might be a bit biased (although I don’t see a penny from any sales), but I think that it and the new edition do the best job of getting people up to speed on serious math of any book I’ve seen, with an introductory “writing your first article” bit and then more advanced chapters covering more advanced mathematical typesetting, basic macro programming, and more. The new edition also covers getting a TeX system installed, picking an editor, and other useful information for getting started.