When Christopher says "best-selling series", I think he means across the entire market worldwide; adding up sales for all Trek books collectively rather than considering any one specific country.
Though what is the top-selling series in Germany if not TOS, out of curiosity?
TOS, best-selling at large, point taken.
According to Cross Cult Destiny was a hit. Vanguard was great. TNG is good, also Titan. New Frontier and Voyager are all right. ENT had also bad numbers of sales, so they decided not to release follow-up novels (Christopher´s Rise of the Federation for instance). I´m fine with that, as I´m no ENT fan.
I don´t know about Typhon Pact. I liked this whole series.
Markus Rohde could offer more precise information, if he´s around again.