If you want a great point to "test the waters" without committing to an ongoing series, as well as something with the original cast, I recommend Christoper L. Bennett's Ex Machina, which I just finished reading (about four days ago). A great story that ties seemlessly into canon, has something for everyone in the cast to do (without just dropping someone in for the sake of seeing them), and stands alone as a great Star Trek tale.
If TOS is your thing, I'd then recommend either moving into David R. George's Crucible trilogy or the Vanguard series, both of which offer rich, expansive tales set in the "classic" timeframe. I also recommend Federation by the Reeves-Stevenses - probably the best Trek novel ever written (one of the few tie-in books that I've read that actually matches the source material in excellence).