With The Guns
This book was published in 1916. The author apparently preferred to remain anonymous; we presume that he was an artillery officer. The book is about the science and art of gunnery as it was developed on the Western Front, mainly centering around the Battle of Loos - so quite early in the war.
The book is written in the style of the time, but it appears to be a pretty accurate description of what actually went on, so it should be regarded as a quite valuable record.