Poul Anderson’s 1974 A Midsummer Tempest is an alternate-history fantasy novel.
Lights up on a familiar scene. The Royalists are trounced by the Roundheads at the Battle of Marston Moor. Royalist commander Prince Rupert flees the debacle but is captured by Shelgrave’s forces. While capture does set-up a meet-cute between Rupert and his captor’s niece Jennifer Alayne, it seems the end of Rupert’s career is at hand.
Despite similarities, there is a crucial difference between this Battle of Marston Moor and the one we know. In this world, William Shakespeare is not the great playwright. He is the Great Historian, whose works record events that actually happened.