I'd consider that a core rulebook though. I wanted to omit core rulebooks because otherwise it's just a "my favorite edition" post and I wanted to focus on other often overlooked products.Rules Cyclopedia. The best one and done D&D book you can buy.
BECMI: The Grand Duchy of Karameikos Gazetteer
How does the Karameikos Gazetteer compare to the material in the 2e boxed set?B/X. Grand Duchy of Karameikos.
It didn't include the Immortal rules though - so not quite one and done!Rules Cyclopedia. The best one and done D&D book you can buy.
I don’t know that I’ve ever seen the boxed set.How does the Karameikos Gazetteer compare to the material in the 2e boxed set?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.