Study guides for writing a good ending are a bit more elusive than craft guides for writing a snapping beginning. I polled the professionals and discovered there’s no real go-to source dedicated to endings.
One book that a friend did recommend is Beginnings, Middles, and Ends by Nancy Kress. Specifically for fiction writers, this book discusses the mysterious ending.
What did everyone else recommend? Read some good endings! Pick up some classics/critically acclaimed books and see how they did it. Study your genre. If you’re thinking about pitching to a specific publishing house, study their books in your genre (some houses want their romance novels to end with a proposal – does yours?).
I wish I had more concrete information for you, but this is part of the fun of novel writing. There are rules and there is freedom. Contradictory, but fun!
Do you have any recommendations for endings? What resources have you used?