Call of Cthulhu, 6th edition
The sixth edition book is badly organized in the areas that like explain how to play , but it has a superb appendix that is the best of its kind in any ttrpg book I've seen. It also has a unique chapter " de rerum supernatura " which includes stuff like, "cthulhu" spelled in Greek, Arabic, shit like that . fucking Iconic shit . It also has 4 , 4 fucking adventures ! and usually only costs 10$. Another thing the 7e core omitted was the personalities bit. That gives you stats for figures such as Randolph carter, nahab , etc. It is essentially indispensable even with the 7e book. The spell section is much more possibly too robust , and is a clusterfuck cause there's no index in it such as in, for example, the 3.5 DND core. The section on the evil books is longer, not much longer, but it is much better. The actual system in this edition was legitimately rough.