I have more than enough Trek to keep me busy for a very long time and I'm crazy for asking this: Are any of the Original Series books worth reading and what do you recommend? I'm only interested in reading one or two of the best because I'm guessing the quality is even more varied than the show itself.
There are certainly more than two worth reading. There's also a forum here dedicated to Trek Literature where you could get all of your questions regarding the book line get answered by the experts, so to speak.
Personally, I'd highly recommend "Ex Machina" By Christopher Bennett which is a sequel to the first movie and the TOS episode "For the World is Hollow and I Have Touched the Sky" - even if you end up being underwhelmed by the first movie.