Gerda malaperis! ("Gerda Vanished!") is a novel written in Esperanto by Claude Piron of Switzerland and is one of the most famous stories in Esperanto literature.
In it, dialogue begins very simply and progresses in difficulty using everyday vocabulary (with word-lists available in various languages). It's suitable for beginners trying to improve their grasp of the Esperanto language, a rehearsal of basic grammar and an entertaining story as well. The mystery begins when three college students witness the strange disappearance (capture?) of a woman who is an expert in "cryptic languages"...
Originally published as a book, "Gerda Malaperis!" has been incorporated into electronic courseware and a movie is now available on DVD.