Try the triology: Star Trek: The Eugenics Wars.
The first two tell all about the Eugenics Wars and Kahn. The third tells the story of what happened between the Kahn episode on the original series and the events in Wrath of Kahn.
All by Greg Cox.
Oh that's easy, look for any of star trek books written by Dave Stern
Daedalus' Children Those two should be read in order Daedalus and then Daedalus' children
What Honor Price
Those four are set in Enterprise, He has a book set in orginal series but I can't remember the title at the moment
Also the Genesis Wave series was good too
John Vornholt wrote them
I think there's four of them
There set in the Next Generation era
Best luck to you
And boldly go where no reader has gone before