Hi BritishSeaPower, I'm doing a similar thing - at the moment I'm up to Krad's Klingon Empire book.
As for the mirror universe stuff: Just for a completist point of view you could also watch TOS: 'The Tholian Web' before ENT: 'In a Mirror Darkly' to get the backstory of the Defiant. Also Stargazer: 'Three' features Gerda Idun Asmund (from the mirror universe) who is in 'The Traitor' (chronologically 'Three' occurs a few years before 'The Traitor', just after 'The Sorrows of Empire'). Talking about 'The Sorrows of Empire'; if I'm right in thinking 'The Black Flag' is placed half way through the story and can be read inbetween pages 162/163. David Mack might be able to clarify this though.