The best mystery book (or book series) of all time would have to be the Sherlock Holmes stories. They are all surprising and detailed, plus, you can make pretty good stories based on them. The one I like the most would be "The Speckled Band". I read it for the first time in 4th grade for a school project. I haven't read many others since, but from the ones I have read, they are a a really good series.
For the mystery story in language arts, I am making a 'Kerlock Holms' story that is pretty good. I think it has a good base for it, that can be changed into numerous outcomes. First of all, Kerlock finds out that there is a zombie in England. Then, he finds out that there is a zombie scientist who went to the same high school as the victim. Surprisingly, the zombie scientist dies, therefore eliminating the prime suspect. From there, you can make anything happen. You can a great story out of that. That's why Sherlock Holmes books are so good, they can give you ideas for a great story.