“The Horse’s Mouth,” by Mervyn Millar, recounts the development of “War Horse” for the stage. It might be too technical for the general reader, but it would be interesting for any fan of the book who’s curious about how it got turned into a play. Among other parts of the process, the director, playwright et al. visited farm horses in Devon, where Joey’s story begins, and the London home of the King’s Troop Royal Horse Artillery, where training still has much in common with its WWI practices.
The book also is notable for what the various members of the theatrical team had to say about the war. Most admitted they didn’t know much about it when they began, beyond what they learned at school (which in the U.S. would be nothing).
And here’s Michael Morpurgo on what played into his inspiration to write the book in the first place: