My family and I love the Beastologist books by R.L. LaFevers, better known by her teen works as author Robin LaFevers. They are about a young man named Nathaniel Fludd, his search for his parents and his apprenticeship as a young beastologist, or person who cares for magical creatures. They are quick reads--we have read entire books aloud in the car in a single day between errands before--and they are filled with magic, creatures and adventure.
Sadly, after we finished the fourth book in the series, we learned that there are no more books! The author left the series on a cliffhanger, which is so unfortunate, but she says that between her publisher saying they won't sell and all of the work it takes to complete her YA fiction, it's just not in the cards right now--and hasn't been for years. I remember reading several series that ended in similar ways as a kid and being so disappointed.
It's really too bad, but I think we still would have read them had we known because they were still a fun set of books. Have you had a similar experience with a book series? Today it's easier than ever to research and find out if a series is complete or not, but sometimes we just take it for granted and don't find out until we search for the next book!