by Pat Barker. Dutton/ William Abrahams, $20.00. Ms. Barker’s novel is a subtle, highly convincing blend of historical fact and imaginative reconstruction. The fact is the presence of the poet Siegfried Sassoon in a hospital for shell-shocked soldiers of the First World War and his treatment by the neurologist Dr. William Rivers. The reconstruction is Ms. Barker’s view of the interaction between Sassoon and Rivers, and of the characters and the miseries of other patients in the place. The situation was, and remains, provocative because almost nobody thought Sassoon was actually off his head. He was a capable, decorated officer and well known for courage when he finally lost patience with the continuing carnage of trench warfare and made a highly public complaint about what he had come to see as aimless slaughter. Rather than resort to military discipline, which would have been embarrassing in conjunction with his record, the authorities declared Sassoon “mentally unsound” and hustled him off to Craiglockhart Hospital, where he partially undermined the official certainties of Dr. Rivers. It is that process which Ms. Barker explores and converts into a remarkable and impressive novel of war and its psychological side effects.