I've started working through the TeX Book for educational purposes about a year ago. It obviously lays the foundation for a deep understanding of TeX/LaTeX. If you're still getting surprised by syntax or the macro system in general, you can skip the chapters which go into minute detail and still take a lot of interesting things away: layouting commands, page structure, units, registers, understanding memory limits and allocation, etc.
That being said, LaTeX has its own kernel, on top of which comes a document class, on top which comes any number of packages. Each layer can alter or mask the lower layer, so understanding LaTeX on all its levels is a difficult task. Then, there are different implementations which add fundamental aspects to the Knuth reference TeX, like pdftex, luatex, xetex, and that does not even include platex, uplatex and other special things.
The little things I know I learned by finding a package/class/command that does what I need, and then study how it does it. E.g. hyperref is a very interesting starting place (relates closely to how documents (DVI, PDF,...) are output), as is memoir or my favourite, microtype.
That being said, LaTeX has its own kernel, on top of which comes a document class, on top which comes any number of packages. Each layer can alter or mask the lower layer, so understanding LaTeX on all its levels is a difficult task. Then, there are different implementations which add fundamental aspects to the Knuth reference TeX, like pdftex, luatex, xetex, and that does not even include platex, uplatex and other special things.
The little things I know I learned by finding a package/class/command that does what I need, and then study how it does it. E.g. hyperref is a very interesting starting place (relates closely to how documents (DVI, PDF,...) are output), as is memoir or my favourite, microtype.