Planescape is a campaign setting for the Dungeons & Dragons fantasy role-playing game, designed by Zeb Cook and published in 1994. It crosses numerous planes of existence, encompassing an entire cosmology called the Great Wheel, as developed previously in the 1987 Manual of the Planes by Jeff Grubb. This includes many of the other Dungeons & Dragons worlds, linking them via inter-dimensional magical portals.

Sources: Planescape - Wikipedia

If you are interested in the suggested reading list located in the back of the 5e Players handbook, check out this reading challenge.  Dungeons & Dragons Player's Handbook Inspirational Reading | The StoryGraph

If you are interested in other campaign settings novels, check out these reading challenges: