Amazing introduction for beginners and experienced GMs alike. If you can pay for one RPG book, choose this one. See also his Character Backstory Guide for players.
Buy on Amazon: Paperback, Kindle ($11).
A set of essays on world-building for a campaign. Originally published in Dragon Magazine from 1997 to 1999, few years later collected online by John H. Kim. In 2020 I've formatted the essays for reading on e-books.
One principle that every game master should be familiar with, referenced from hundreds other articles.
Get the A4-sized 317 page book by subscribing to a newsletter, or read the original article.
Kobold Press produced multiple essay collections on game design, plots, gamemastering, and other things. Two books are universally appraised:
The Alexandrian is the richest source of information for adventure design and running games. I plan to produce an unofficial e-book with the core essays.
Another huge archive of articles with player and DM advice, lists and of course much on adventure design. Very focused on D&D and OSR.
Blog posts available on DTRPG in four PDF compendiums ($3—$5 each). Not exactly formatted for smaller screens, but it'd be unfair to make a free re-formatted copy.
This designer of the GUMSHOE gaming system has been writing an RPG advice column for the Pelgrane Press' See Page XX monthly webzine, and his first articles were deemed influential enough to be removed from the web into a paid PDF. Not sure if they are worth it, since pages are again A4-sized.
You can get the PDF on DTRPG for $4 or here for $3 (click "Other Products" → "PDFs").