The film Regeneration is based on the book Regeneration by Pat Barker. Both are good, although I recommend you read the book before you see the film. Both deal largely with the lives of several people in a First World War army hospital dealing largely with shell shock, with the main character being Dr. Rivers, one of the heads of the hospital, a converted country house. Other important characters include the war poets Sigfried Sassoon, Robert Graves and Wilfred Owen, as well as several other officers and staff.
The most major difference between the film and the book of Regeneration is the length of the film: several parts of the film have been edited to a length that is more suited to cinema, and the shorter concentration spans of cinema audiences. The section of the book where Rivers visits Burns and Burns relapses is cut, as is much of the dialogue between Sassoon and Graves, and most of Priors romance. Several major characters in the book, are hardly seen in the film.
The nature of the medium of film does enhance much of the story, particularly the flashbacks to the trenches. The music in the film is used to good effect, with the slow booming of artillery often used, however the flashbacks do not do as much to reflect the horror of the trenches as the book, and some of the books other themes, such as class division, are hardly covered at all in the film, and the cutting of the plot leaves it oddly disjointed at times. The film is a fairly accurate representation of the book, and the plot had not been greatly changed, but it is too short to really do the book justice.