The Way Through the Woods — Colin Dexter

Morse is enjoying a rare if unsatisfying holiday in Dorset when the first letter appears in The Times. A year before, a stunning Swedish student disappeared from Oxfordshire, leaving behind a rucksack with her identification. As the lady was dishy, young, and traveling alone, the Thames Valley Police suspected foul play. But without a body, and with precious few clues, the investigation ground to a halt. Now it seems that someone who can hold back no longer is composing clue-laden poetry that begins an enthusiastic correspondence among England's news-reading public. Not one to be left behind, Morse writes a letter of his own--and follows a twisting path through the Wytham Woods that leads to a most shocking murder.