These resources may be of interest as supplementary materials for independent learners or to people teaching LaTeX to others. Since they are materials prepared for workshops, they are not intended as standalone or self-sufficient materials for people learning alone. However, some of the material may be of use to people teaching themselves, if used in conjunction with one or more of the comprehensive introductions featured in other answers.
The materials are prepared for workshops taught for Cardiff University's Doctoral Academy and are updated from time-to-time. However, it seems impossible to edit an Overleaf project once published. For obvious reasons, I'd also prefer everything to be in the same place, so I hope to migrate LaTeX I to GitHub next semester.
Slides, notes and exercises adapted from UK TUG's introductory training course and prepared for an introductory postgraduate workshop:
Supplementary handouts provided include the LaTeX cheatsheet (http://www.stdout.org/~winston/latex/) and David Carlisle's list of maths symbols (https://ctan.org/pkg/maths-symbols).
The original UK TUG project maintained by Joseph Wright:
Slides, notes, exercises and supplementary handouts prepared for a series of four intermediate postgraduate workshops:
The workshops cover
custom macros, packages and classes;
bibliographies with BibLaTeX and Biber;
presentations with Beamer;
float management, use of functions and external data files with
pgfplots and drawing with TikZ.
The notes for these workshops all include appendices featuring various kinds of additional information and topic-specific pointers to further resources. For example, the first workshop itself introduces only the core LaTeX 2e facilities for creating commands and environments, but the appendices introduce resources covering some of the more advanced options provided by TeX and
Supplementary handouts include a Biblatex/Biber cheatsheet (https://ctan.org/pkg/biblatex-cheatsheet), a font sampler and annotated lists of general, discipline-specific and font package recommendations. The sampler and package lists are included in the project repository (https://github.com/cfr42/latex-2/tree/master/handouts).