The Prisoner in the Castle is the eighth in the Maggie Hope series by Susan Elia MacNeal. It has dropped to $1.99 at Kindle US as part of the Daily Deal, matched at Kobo US.
Kindle US:
https://www.amazon.com/Prisoner-Cast...dp/B077CQDZFC/
Kindle US/Smile:
https://smile.amazon.com/Prisoner-Ca...dp/B077CQDZFC/
Kobo US:
https://www.kobo.com/us/en/ebook/the...-in-the-castle