Last update 1 February 2011.
Female Intelligence: Women and Espionage in the First World War by Tammy Proctor. The most useful thing I learned from this book was how the underground spy networks in Belgium were organized, and some specific stories about women who worked in intelligence-gathering, and their fates. I also extensively used this book's bibliography to locate other sources.
1915: the Death of Innocence by Lyn Macdonald is an excellent collection of first-person accounts.
The Long Silence: Civilian Life under the German Occupation of Northern France, 1914-1918 by Helen McPhail is the most useful book I found. The author points out that it's not a thorough academic study, but there was more than enough information for my purposes. I got a really good feel for what life was like in occupied Northern France, as well as the various problems and subterfuges of the people living there. I'm hoping the information in this book will help me to vividly describe the place and time without directly copying actual events.
The Belgian Army in World War I is an Osprey book. These are slender but dense with detailed information and drawings.
In Flanders Flooded Fields: Before Ypres There was Yser by Paul Van Pul. This is an exceedingly detailed and thorough book on the deliberate flooding that stopped a German advance in 1914.
French Women and the First World War: War Stories of the Home Front by Margaret H. Darrow is a more academic book that gave me a lot to think about thematically, as well as providing factual information.
This is a collection of links to books I used for research while writing The Moonlight Mistress.
Not a book, but this detailed black-and-white documentary proved to be amazingly useful simply to look at real people from the period in motion: The First World War - The Complete Series. It's the best documentary I've seen so far on the war.
I used this book for information about wolf behavior: Of Wolves and Men
A good introduction to World War One, originally published as a tie-in to a television documentary: The First World War
One of the best known and most readable books on the early days of World War One: The Guns of August
This is a much more detailed, academic work on World War One as a whole: The First World War
An excellent photo reference: World War I: A Visual Encyclopedia
Probably not necessary for the general reader, but I found it really interesting: The Palgrave Concise Historical Atlas of the First World War
The single most useful reference I had for basic information about the British Army during World War One: Tommy: The British Soldier on the Western Front 1914-1918
This is a collection of first-person accounts, woven together consecutively. Most of my detail about the first battles of the war came from this book: 1914
A spectacularly useful reference for the day-to-day life of a nurse and her state of mind in the war's early stages: Diary of a Nursing Sister on the Western Front: 1914-1915
More first-person accounts from nurses, doctors, VADs, and other medical personnel: The Roses of No Man's Land
The Osprey books feature photos, drawings, and in-depth information on uniforms, equipment, and tactics: The French Army 1914-18
"Jam tin" grenades came from this Osprey book: World War I Trench Warfare (1): 1914-1916
One of the most well-known books on the poets and artists of World War One; most of it addresses the later years of the war. Frequently referenced in later works: The Great War and Modern Memory
An academic study of male friendships in this period: Modernism, Male Friendship, and the First World War
This is possibly my favorite book on World War One to date. I'd recommend reading it after reading some more general references for context: Rites of Spring
This is a collection of writings by women from both sides, all over the world, on the war itself and on pacifism: Lines of Fire
Further Tactical Reading
Detailed information on the French army, if you're interested in military history: Paths of Glory: The French Army 1914-18
Focuses on Ypres: A Storm in Flanders
More on Ypres: Ypres: The First Battle 1914