Unlike Regeneration, which can function as a stand-alone novel quite apart from its place as the first in the trilogy, I think both the subsequent books require the knowledge of the characters and the circumstances that comes with Regeneration. Billy Prior, who has somewhat of a secondary role in Regeneration, as opposed to Rivers and Sassoon, takes centre stage this time, and despite being one of the few fictional characters in this trilogy, is arguably the most fascinating.
Prior is a working-class officer, working in Intelligence when he longs to be back at the Front, investigating anti-war pacifists, most of whom he grew up with as a child, bisexual, neither fish nor fowl and the strains of this shatter his psyche and he suffers from memory lapses, blackouts and even a split personality. He's a wonderful, brittle, hard-edged character, eminently memorable, and a heartbreaking example of the inner wounds war can inflict on even the strongest of characters.
Many people make the mistake of thinking of this trilogy as a 'war trilogy' and that does it a disservice, almost. It's so much more. The Western Front only makes a physical appearance in the final novel - this is a trilogy about the mental scars of war, about the pressures of government and politics during war, about the evolution of mental health care, about sexuality and national pride. I think this is my favourite book of the trilogy.