Recs if you like My Little Pony: Friendship is Magic Volume 1...

how to get access to many books by reading one book (series)

aka. books with the most works on ao3 about them (fandoms with 10k+ works, in no particular order) (sometimes the fanfics may be about a movie adaptation rather than the specific book but still fits in the overall fandom)

A few excellent graphic novels for kids! These books are mostly for Ages 6-15, but some vary. Double-check first before recommending them to a kid, you don't want to accidentally give a book that's too hard to read for them. It's already hard enough to encourage their love for reading without making it harder than it has to be! But, these are all still very good for reluctant readers.

Books that focus on mythical creatures besides dragons or humanoids. Only includes the first book in each series. Feel free to offer suggestions in the comments.