It is available free as a Kindle ebook: https://www.amazon.com/Over-Top-Arthur-Guy-Empey-ebook/dp/B0... (some reviews can be found here: http://www.goodreads.com/book/show/1517707.Over_The_Top)
Empey was an American who managed to get in the mix with the British soldiers before America entered the war. His account is based entirely on his own first-hand experiences. His last patrol depicts an utterly futile foray at night in which dozens of his comrades lost their lives and he (and I believe one another) survived. I am going from memory here on something I read quite a while back and so my summary may be imprecise. But anyone wanting to get a sense of the horrors of what it meant to fight in such battles will most certainly get it through this account.
The strange thing is that this became a runaway bestseller in America in 1917 and 1918. Far from recoiling at the horrors, the public chose to celebrate the heroism of the soldiers who put themselves in such incredible danger.
It is in any case a bone-chilling account to read this and all the more so because its style is almost detached and matter-of-fact, even clinical in describing one horror after another.