Not quite what you’re asking for, but:
Princess Elizabeth’s Spy: A Maggie Hope Mystery
by Susan Elia Macneal
I’ve been reading this (slightly cheesy) pleasant series of Maggie Hope mysteries for several years now. This particular one is set in Windsor Castle, and it seems well-researched, with appealing details, and familiar (!) characters.
"If you look for the bad in people expecting to find it, you surely will.”