I've just returned to the New Earth series, which is set between The Motion Picture and Wrath of Khan. I never finished the third book when it came out. On rereading, the first and second are excellent, and TOS Trek really at its finest. I can see why I got bogged down in the third, but I'm about finished and it's outstanding too.
I'm afraid that I don't much care for the current state of affairs in Trek Lit. Lots of people absolutely LOVE the Titan
series. If you're a Riker or Troi fan, it may be for you.
I am a fan of the Vanguard
series, but it's not the main Trek characters and it's set early in TOS's run.
Otherwise, I really think that Trek Lit's hayday was in the 8 years between The Motion Picture and the start of TNG.