I do not understand this. Why is Martha -- the companion in only one series -- in the same amount of books as Rose -- the companion in TWO series. Furthermore, why is Martha in THREE times the number of books that Donna -- also the companion in only one series? I don't get it. I thought at first that maybe the popularity grew so much that by the time Martha came around, they realized there was a bigger market and so more books and that more Donna books would be coming. However, books are started to coming out (or will this year) of the Doctor alone, post-Donna. So I don't get it.
And on not getting it, why can't there continue to be Rose/Nine and Ten/Rose and, yeah, Ten/Martha, Ten/Donna books just because the actors are gone? They can still fill in adventures with them that wouldn't contradict what's on the show.
Anyhoo, just wondering.
ETA: Well, after reading responses I imagine it probably has to do with image licensing. Both Billie and Catherine Tate would know how to milk that stuff in a contract and Freema wouldn't. Ah well. Sucks, that means it's likely we won't ever see anymore Nine/Rose, Ten/Rose, Donna/Ten or any TenII/Rose period, books. Sigh.