TROIKA! (Science Fantasy RPG) is free if you join a mailing list. The 144-page bookmarked PDF is rules + an adventure. Portal, labyrinth or barge lets you encounter goblins, dragons, living dead, etc
TROIKA! is a science-fantasy role-playing game where you and your cosmopolitan group of fellow travellers explore the ends of multiverse. You will fly on mystic barges, help dying gods, solve confounding crimes, plunder dead worlds, and meet strange & wonderful people.
With just one book...
The title is heavily edited for the main ideas of the game. Here is the blurb from the PDF:
...what you have here is Troika!, a science-fantasy
RPG in which players travel by eldritch portal and non-euclidean labyrinth and golden-sailed barge between the uncountable
crystal Spheres strung delicately across the Humpbacked Sky.
What you encounter on those Spheres and in those liminal places is anybody’s guess – I wouldn’t presume to tell you, though
inside this book you will find people and artefacts from these
worlds which will suggest the shape of things. The adventure
and wonder are in the gaps; your game is defined by the ways in
which you fill them.