Retreat, Hell We Just Got Here!
There are not many books about the participation of the US forces available. This one, from the Osprey Special Editions, deals with the entire history of their actions in brief. A great number of photos, maps and the usual Osprey Illustrations from the Men-At-Arms series complete this work.
ISBN 1-85532-777-5

Atlas of World War I
If you don't have one, get one! A WWI Atlas is the best addition to most WWI history books. This is one of the best I've seen. The actions are taken chronologically and are covered on two pages each with colourful maps and short history. The introduction and summaries of the different war years are among the best I've read about WWI.
ISBN 0-670-85372-0

Battle Europe Series
A series, Tim and I discovered last time we went to Amiens. Almost the only books today available that deal with single actions of the big offensives. Obviously "British emphasized" and mainly a battlefield guide, the different authors are mostly keen battlefield walkers, who know every pile of debris, and try to give a impression of both sides of the lines. There are plenty of wartime photos and more importantly some aerial pictures (although of poor quality) with atmospheric eye witness accounts in about 140 pages books. There are 10 books of the Somme sectors alone, their focus now turning to the Ypres actions and other battles. Two battlefield guides in their own respect are covering the Somme and the Ypres Salient. Have a look at http://www.yorkshire-web.co.uk/ps/battgrnd.htm

Men At Arms, Campaign, Warrior - Osprey Books

Osprey seems to be the leading publisher of military books especially in uniform matters. While the Men-At-Arms Series are, after a number of decades, numbered in the 400s covering different eras and uniforms, the Campaign and Warrior series are now also important parts of their program.
Each MAA has about 50 pages with 8 coulour plates, mostly made by famous illustrators. These books are wargamers and modelers first choice. The information is ample and in many cases the only one you can get about a certain era. The range begins in ancient Egypt to modern armies.
The Campaign series was thought as a quick reference for Wargamers, but now covering battles you might never heard of. Battles of Alexander, Hannibal, 100years war to Napoleon, Scherman and Patton are covered with contemporary pictures and photos together with maps and birdseye views. Using MAA plates at the start they are now exclusively illustrated.
The Warrior series is sometimes regarded by reenactor as their making instruction manual for their gear. While MAA are mostly only dealing with uniforms these books take a look the man in them together with their equipment and background. This series is now one the best illustrated. The information about tactics and background is good. There are two titles dealing with the German Stormtrooper (now also in a special hard cover edition) and the British Tommy.
Have a look at http://www.osprey-publishing.co.uk/

The Tanks at Flers
A two volume big work about the first tank action ever. The author managed to cover the story of the British Mark I tank from the construction and training to the action at the Somme in September 1916. The book is full of unit accounts of the action and you can retrace the tracks of nearly all (49) tanks and crew which were went into action. A great number of wartime photos aerial pictures are completing this report, but nicest thing is the 2nd book, containing handout b/w facsimiles of frontline maps showing the offensive movements and trench lines.
ISBN 0952517507

Krieg
Ludwig Renn was the pseudonym of Arnold von Golssenau who's autobiographical book is of equal intention as "All quite on the Western Front" but never got that famous. The entire war can be seen trough the eyes of a company commander. The style of writing is sober and unpathetic and in case of German post war literature one of the most important "war enlightenment books". Contrary to Remarque´s book one can follow the author's way from the "Vormarsch" through Belgium to France by the location names given. His experiences of the Somme battle is worth comparing with those Jünger made. I'm sure that it was published in English, therefore: Highly recommended.

Linnenk
One of the best German book for the understanding of weapon technology and tactics of WWI. Interesting to read is that there were a number of people on all sides before the war who analyzed the Russo-Japanese war and predicted exactly what had happened in 1914 to 18, Sickly enough nobody listened then. Moreover you get a notion that the soldier of that time received the highest technology weapons available and did the best they could do with it. It covers nearly all kinds of infantry weapons, artillery systems and their development before and through the war. CC2 and CC3 The Great War would be nothing without this book! Sorry, only in German (but, hey, this is an international page, isn't it?).
ISBN 3-7637-5966-2

Machine Guns of World War I
A very nice photo book about the major emma gees of WWI. These battlefield survivors are in the hands of certain WWI reenactor groups and serious collectors in the US. Photos were shot on a recreated WWI battlefield area where the guns were fired with live ammo. The beautiful photos show every detail of these mass killing machines while in position and dismantled. The texts are full of information about their usage you hardly find somewhere else. Guns covered: Vickers, Lewis, MG 08, MG 08/15, MP 18/1 (!!) , Hotchkiss and Chauchat.
ISBN 1-85915-078-0

World War One Source Book
This tireless author is more known for countless books about the Napoleonic wars. The book is a complete work about nearly all matters concerning the Great War. Covering technology and tactics in short it's main part consists of a good summary of the main actions on virtually all fronts together with a description of all warring parties (Did you know that Siam declared war on the Central Powers?), Biography of the war leaders and a voluminous Bibliography. It's a good start to get into WWI matters.
ISBN 186019 852 X

The Price of Glory
The classical English work of this famous historian about the Verdun battle. Already published in 1962 it was well researched with many eyewitness accounts. Although many new aspects were discovered since. So you still can read the false casualty numbers and the untrue story of the first German who entered the Fort Douaumont. Still, worth a read and to begin own researches.
ISBN 0-14-017041-3

VerdunWerth
The most accurate German book, written in 1982, about that battle and in my opinion more then equal to Horne's book. Naturally enough it emphasizes the German view of the actions but since it is written by a Germanic contains a number of side studies about myths, post war literature, Prussian militarism and especially German perception of this infamous onslaught. The author had the chance to interview a number of participants and their records make this book exciting to read. Don't know if published in English, but it should be, and so, highly recommended. ISBN 3-404-65041-7

Wäld125
Another book of Jünger, who died in 1996 at the age of 104. This book is quit awkward to read, as it contains what "Storm of Steel" does not: a good deal of nationalism and glorification of most intellectual quality. Actually the foreword is indirectly aimed against H. Barbusse and the pacifists' movement. Nonetheless it is interesting to read of the action around 8th August 1918, in the Somme sector as it is told in his usual, emphatic style. In the meantime you got the impression of how the daily routine of a frontline company was held and how a company officer led his men. All this descriptions are mixed with long and bitter monologues of the things mentioned above. Surely published in English, suggested for careful readers only!