The book is, of course, excellent -- a story told in the style of Black Beauty, from the horse's point of view and starting from his early days as a colt. It also has something of a message -- not just how horses should or shouldn't be treated, as Anna Sewall was trying to do, but how terrible and pointless war is.
When I was reading the book, I wasn't thinking about its anti-war message, but then I read a review of the movie that mentioned it, and it occurred to me that yes, the book was rather anti-war too. There is a scene toward the end of the novel where one soldier from each side has come forward to claim Joey (the horse), and as the two of them are trying to decide who will take him, one suggests flipping a coin for him. Then he comments on how much faster the fighting would end if the two of them were allowed to negotiate the end of the war the same way. Boiling the entire conflict and its outcome down to the flip of a coin demonstrates how unnecessary all of the death and destruction is.
Without getting into a lot of detail about the war and the reasons behind it -- the book is, after all, told from the point of view of a horse, and what do horses care about politics? -- War Horse is well-researched and authentic, not to mention a poignantly beautiful story. What a great read -- I can't wait to see the movie!