Short RPG books, many about worldbuilding but also scenarios, monster books and so on. Most are currently available digitally only. If you want to get hardcopies, why not ask your favourite zine seller? It could be arranged.
Pamphlet of Pantheons, my system-neutral guide to creating religions in fantasy games.
Guide to Grunts, my system-neutral guide to creating armies and soldiers in fantasy games.
Tome of Tombs, my system-neutral guide to creating burial sites in fantasy games.
The Magonium Mine Murders, my crime caper / murder mystery adventure for Old-School Essentials.
The Budget Bestiary, my guide to making monsters from found objects and army men. With stats for Old-School Essentials. Free/PWYW.
The Ghoul Gallery, a free/PWYW collection of monsters for the RPG Brave Zenith.