Patricia Finney writes the Sir Robert Carey series under her pen name PF Chisholm. I am quite fond of that series and have enjoyed it a lot over the years. She also has written a different short three-book series, featuring Simon Ames and David Becket, set in roughly the same time frame, using her real name.
Firedrake’s Eye is the first in this secondary series, and it is free right now at Kindle and Kobo US. And, the last time it was free, it also was free at Kindle UK, so although it’s £0.99 there right now, it might be worth checking again at some point in case it drops later.
Also note that if you like this one, and want to read the remaining two, there is an omnibus of all three that has been on sale occasionally in the past, and might offer a better price if you are patient, even if just for two of the three titles. That omnibus can be found at: Kindle US | Kobo US | Kobo UK