Birdsong by Sebastian Faulks (1993) — The best-known English novel about the Western Front, partly set in the Ypres Salient. Faulks captures both the horror and the strange domesticity of trench warfare with unusual emotional intelligence. The passages describing the tunnel miners are unforgettable.
Ypres: The Holy Ground of British Arms by John Hussey — A detailed history of the Salient from 1914 to 1918, comprehensive without being dry. Best read before visiting rather than after, to understand the geography and the chronology of what you're walking through.
In Flanders Fields by Leon Wolff (1958) — A narrative history of the Third Battle of Ypres (Passchendaele) written with journalistic clarity and genuine fury at the command decisions that produced so many casualties for so little ground. Harder to find than Faulks but more illuminating about the specific battles you'll be standing on.
The Wipers Times edited by Ian Hislop and Nick Newman — A facsimile edition of the trench newspaper produced by the Sherwood Foresters in the ruins of Ypres (they nicknamed the city "Wipers" in the cheerful deliberate mispronunciation common in the British Army). Satirical, dark, and very funny — an important corrective to the idea that the soldiers spent the entire war in despair.