Found: mystery/suspense, guy follows a deer to a castle on his property that is not on the map

A guy buys a house and property in rural Pennsylvania (?) and experiences spooky things as he works on it. He follows a white deer into the woods and discovers a part of his property that is not on the map and a hidden castle. In the end he begins to wrestle with experiences he had as a soldier in war.

It doesn't fit all your particulars (set in England, castle?), but it brings to mind Mythago Wood by Robert Holdstock. The main protagonist has just returned from WWII and enters a magical wood near his childhood home.

Yeah, that sounds really similar in some ways, and I wonder if this was an inspiration for the book I'm looking for. I think the missing book was written between 2005 and 2015 and though it is speculative with supernatural/magical elements, the story has a very realist feel to it (not fantasy at all). So it would be very interesting if the author took Mythago Wood as a scaffold to do something new. Thanks for the suggestion.

Check out Castle by J. Robbert Lennon!

That's it! Thanks!