Just started re-reading the Errend of Vengeance trilogy by Kevin Ryan, which sees TOS Enterprise thru the eyes of an innocent and naive young Klingon who is undercover as a spy as a redshirt on the Enterprise. A Klingon posing as a Starfleet crew member as the lead is a lot of fun, and getting his take on everything gives great insight to the Klingons of the 23rd century. Will be reading the sequel trilogy Errand of Fury as part of the larger re-read following Errand of Vengeance.
I read Double, Double decades ago, and remember that I liked it. I still have it and plant to re-read it soon too as part of my massive Trek re-read.