Greg, you just made my day.
I love that we've been having more TOS novels lately. For a good number of years, classic TOS novels were almost entirely missing. I was starving for them, and I'm really enjoying the current pattern: 6 months of TOS, then 6 months of the "modern continuity."
I don't know if TOS novels really sell better than the other series, but it wouldn't surprise me. For a lot of more casual fans, TOS is "Star Trek." The newer series have their fans (including me) but there's a lot of continuity to learn.
Beyond that, I'm hoping we get some good DS9 including the Ascendents story finally. DRGIII's recent TOS novel has gotten my hopes up.