While it's technically three novels, The Regeneration Trilogy is one story, and for convenience comes in a one-volume omnibus. Any of the parts could be read on its own—there's enough brief recap that one could be aware of the events of the other volumes without having read them, and as the trilogy is character-based rather than plot-based it won't befuddle anyone who jumps in at the middle. However, to do so would do the story an immense disservice. Read in its proper order, Regeneration forms one of the great war stories of our or anyone's time, an epic that takes place almost entirely on the home front as it depicts the final years of perhaps the largest blunder in military or geopolitical history.
W. H. R. Rivers doesn't carry a gun, but he sees just as much of the effects of World War I as any soldier. That's his job, in fact. He's a psychologist, and his job is to restore the traumatized, shell-shocked men under his care to some semblance of normal life. Among these patients are Siegfried Sassoon, a poet who refuses to fight not because of pacifism but because of the sheer stupidity of the conflict; Wilfred Owen, a sensitive young man attempting to come to terms with his feelings on the conflict through writing; and Billy Prior, an involuntary sadist who's disconcerted by his sexual proclivities and serves as a counter-analyst to Rivers himself. As the war drags on, and more and more of his patients are returned to the front only to be torn to rags, Rivers struggles to restrain Prior from returning to active duty. However, Prior himself finds his self-disgust increasing the longer he stays away from physical harm, and despite Rivers' protestations begins to believe it's his duty to die in France.
A lesser author than Barker would have become bogged down in the "celebrity cameos" of her story, pointing the reader to the historical characters with many a nudge and a wink, and the novels would quickly have become cloying for it. Barker is smarter than this, and treats Rivers, Sassoon and Owen as no more or less than fellow players along with their purely fictional counterparts. A reader who is not at all familiar with the historicity of the characters would never know that they were anything but Barker's inventions (I myself had no idea that Rivers was a real man until I did some research into the novels' background), and this is a good thing. The story within which the characters find themselves would at any rate be compelling even if it were completely fictional. The sheer horror of the experiences that have landed Rivers' men under his care is far better seen in their symptoms and neuroses than it could ever be if simply depicted in the present tense. The significant amount of time spent with "shell-shocked" soldiers is incredibly effective at turning the Great War from a historical abstraction into a concrete reality; they are, of course, suffering from the same post-traumatic stress disorder that is now known and diagnosed today, and the similarities between veterans of the two eras is heart-wrenching. Equally as compelling is the interaction between characters; Rivers and Billy Prior spend the entire trilogy in a game of cat-and-mouse that is never entirely hostile but never entirely friendly, probing each other for weaknesses and explanations and daring the other man to slip first—the fact that this is intended on Rivers' part as a cure makes the game no less a battle. Nearly as interesting is the paradox of Sassoon, a man who considers it his duty to be with his men but refuses to fight in what he considers to be a pointless conflict. Barker uses her men as microcosms of much vaster societal and psychological issues of their day, but never loses sight of them as individuals.
One thing that Regeneration most definitely isn't is a slog—its 900 pages fly by. However, the weight of its material is near-tangible. Many other novels have been written about World War I, by authors whose talent is undeniable. Barker's, I think, is the one that will go down as the definitive one. It strikes a perfect balance between the factual and the fictional, the human and the abstract, the individual and the era. Truly an incredible achievement.