Parts of Forgotten History
take place in this time period, and the novel over all is amazing. The same can be said of the anthology Constellations.
Of the four "Lost Years" books, I liked The Lost Years
. Leonard McCoy, Frontier Doctor was worth a read too.
I would imagine most people around here would suggest Prime Directive
, but I've not read it. Also, I'm told the old novelizations of TAS make some of the poorer episodes much better, and I love some of the better ones already anyway.