Can we include Augustine? If so, Confessions and City of God for me. It's hard to pick just one, so please pardon me for adding more:

Thomas a Kempis, Imitation of Christ
Bernard of Clairvaux, Sermons on the Canticle of Canticles
Anselm, Cur Deus homo
Letters of John Huss
John Wycliffe, Sermons, Select English Works
Thomas Aquinas, Summa Theologica
Bede's Eccleastical History of the English People
Sir Thomas Mallory, Le Morte d'Arthur
The Song of Roland

__________________
Andrew Myers
Husband of Jessica, Father of Jackson, Katie and Samuel
Member, Presbyterian Reformed Church of Northern Virginia
Warrenton, VA USA
Editor, The Matthew Poole Project

"Let your Morning Thoughts, and your last Evening Thoughts, be what shall become of you to all Eternity." -- Matthew Poole