Knuth's book (that is, the TeX Book) is worth looking at. You're right that it won't tell you everything you want to know about LaTeX, and in that sense is like

Knuth's book (that is, the TeX Book) is worth looking at. You're right that it won't tell you everything you want to know about LaTeX, and in that sense is like learning C rather than C++. But (also like learning C before C++) it provides a lot of foundational knowledge that will be useful if go on to read a book specifically about LaTeX, and if you ever need to do something more complicated than LaTeX's defaults allow. For example, it explains how all layout is done with "boxes" and "glue", and the difference between horizontal and vertical mode.