Finstere Gründe.

Blurb

The Way Through the Woods is a crime novel by Colin Dexter, the tenth novel in the Inspector Morse series. It received the Gold Dagger Award in 1992.
The plot deals with the search for a beautiful young Swedish woman who went missing a year earlier. An anonymous riddle, in the form of a five-stanza poem, is sent to the police and the case is reopened. The police ask The Times for help with the poem. Morse and Sergeant Lewis are put in charge of the new investigation.
The novel was adapted for television in 1995: see The Way Through the Woods, as an episode of the Inspector Morse series.