For hose who enjoy the Tolkien books and Harry Potter series, let me suggest some children's series that are worth the attention of adult readers:
1. "A Wrinkle in Time" Trilogy by Madeline L'Engle
2. "The Dark is Rising" Series by Susan Rogers Cooper
3. "Young Wizards" Series by Diane Duane
4. "The Heroes of Olympus" Series by Rick Riordan
These are well-written, and series I have read and re-read. I own L'Engle and Cooper's series and Duane and Riordan are on my "to buy" list. A word about Rick Riordan. He has several series all related, but not on the level of his "Heroes of Olympus" books.
One adult series on my "to buy" list is Gordon R. Dickson's series "The Dragon and the George".