Murder on the Orient Express - Agatha Christie
I was given this book by my class when I changed high school in the early 70s, but it whetted my appetite for Agatha Christie and Poirot in particular. TV may have increased exposure, but the little Belgian sleuth was wonderfully fashioned in the novels and this book in particular showcases the writer's ability to weave disparate plotlines, culminating in a suitably complex options for the 'solution'. Agatha at her best!