QUESTION
What resources (online or otherwise) exist that give good advice on best practices for TeX, etc.? I'm looking particularly for things that assume you already know TeX or LaTeX, and aim to help you achieve better results, with rationales explained. Resources with some specific focus are welcome. I'll start a list with two suggestions of my own so you can see what I mean.
Somewhat to my surprise I haven't found this question asked here already. (Please point me to it if I'm wrong!) Note that what I'm asking is somewhat different from this question and this question, although there's lots of room for overlapping answers.
ANSWER
The Essential Guide to LaTeX2e Usage (l2tabu) explains some of the most common mistakes in LaTeX usage.
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