For me it's a tie between Quidditch Through the Ages and Fantastic Beasts and Where to Find Them. I got the original box set back in the day and enjoyed them so much that I bought the illustrated versions when they came out. I'm a huge dork for in-universe content. While going through a bad breakup I used the illustrated version of Fantastic Beasts and Where to Find Them to plan out a Wizarding version of NatGeo and had lots of fun.