They Called It Passchendaele by Lyn Macdonald (1978) — The best single-volume account of the Third Battle of Ypres, told almost entirely through the words of the men who fought it. Macdonald gathered hundreds of first-hand accounts and assembled them into a narrative of appalling clarity. The battlefield is 15km from Wervik, and reading this before or after a visit to the Ypres Salient changes what you see.
Birdsong by Sebastian Faulks (1993) — The most widely read novel set in this corner of France and Belgium. A British officer's experience of the Western Front, intercut with a love story set in the Somme valley before the war. The landscape Faulks describes — flat, flooded, fought over — is exactly the landscape you cross getting here. Some find the love story overwrought; the war sections are not.
In Europe by Geert Mak (1999) — The Dutch journalist drove across Europe following the path of the 20th century. His chapters on Belgium and the former Western Front are among the most thoughtful writing on what this landscape means and what it cost. A good companion for any journey through this part of the continent.
Flanders by Bike — Various editions from Uitgeverij Lannoo cover the cycling routes of Flanders in detail, including the Lys valley. Practical for route planning and good on local context. Worth picking up in a Kortrijk or Ghent bookshop before you start.