What were you missing from the online tutorials and manuals, that you found well explained in LaTeX books? Any LaTeX book, but I'm mainly referring to The LaTeX Companion by Mittlebach, and the A Guide to LaTeX by Kopka.
I'm working on a book project in LaTeX and the question came up why don't I buy these much reputed books. Until now I learnt what I use from online tutorials and manuals, but these can be tedious or rudimentary in some cases.
So my counter-question is that in what areas do these books really make a difference?