There are some amazing books in the Planescape line, but if you had to have just one, I think my pick might be The Planewalker’s Handbook (1996). It’s a mechanics-light, player facing sourcebook that restates a lot of the core box set in friendly terms and is packed with some of Tony DiTerlizzi’s best artwork for the entire line — there are so many interesting faces in these pages.

Monte Cook is the scribe here and I think Planescape Monte Cook is my favorite Monte Cook. Perhaps it’s just my long-time familiarity with D&D’s cosmology giving me a leg up, but he seems to have a real talent for conveying out-there metaphysical concepts in an easy-to-grasp manner, and he’s at the height of his powers here. He does a particularly good job explaining how magic twists and changes relative to the plane you are on, something central to the setting that the main box makes feel ponderous. Here, it’s still a lot, but you can get a grip on it easier. Overall I get this book in a way that is far deeper than my experience with Manual of the Planes (which this somewhat resembles in presentation) or even some of Cook’s other Planescape books. It’s a great entry point for the entire D&D cosmos.

What mechanics are here are player options — kits, proficiencies, spells, the usual potpourri of 2E offerings. The kits are especially nice. They just take the four core classes and tweak them for existence on the planes — planewalker rogue, planewalker priest and so on. Of course planar warriors are different from prime material sword swingers! It is a weird oversight that this wasn’t included in the original box.