The Great War: A Guide to the Service Records of All the World's Fighting Men and Volunteers by Christina K. Schaefer is a look at World War I and the records available, worldwide, for those who served in that war. Whether your World War I ancestor was from England, the U.S. or even Russia, this is the book for you.
Countries covered in this book include, Australia and New Zealand, Austria-Hungary, Belgium, Brazil, Bulgaria, Canada and Newfoundland, France, Germany, Great Britain, Greece, India, Italy, Japan, Luxembourg, The Ottoman Empire, Poland, Portugal, Romania, Russian Empire, Serbia and Montenegro, South Africa, The United States, and The Neutral Countries.
The back cover states, "With help from dozens of individuals and institutions throughout the world...the author has managed to compile a guide to World War I service records that is not only unique but totally comprehensive."
Besides providing information on records available, the author has also provide a WWI timeline,maps, some history and research tips.
You can check out this book at Google books, http://books.google.com/books?hl=en&id=_bZxHfktABMC&dq=The+Great+War+by+Christina+Schaefer&printsec=frontcover&source=web&ots=C0LUpSft82&sig=nsi5rOelmxyrHCHbxFpQhvZ1Q-U&sa=X&oi=book_result&resnum=1&ct=result.
A good companion to this that would help add to your library of WWI reference books is Uncle We are Ready! Registering America's Men 1917-1918 by John Newman, http://www.genealogy.com/60048book.html. This book is essential to learning more about the WWI draft registration.