Sometimes a book trailer can be an effective way to relate the contents of a novel to a reader. Sometimes not.
Currently I'm reading Maker of Angels by Dean Sault--in between grading and editing and council and everything else. But reading is important to a writer. It's sort of like an engineer who designs cars saying he doesn't have time to take an automobile out on the road to just enjoy.
Anyway, here's an example of what I would say is a well-done book trailer. And for the record, what I've read of Maker of Angels thus far? Well worth my reading time.