The books released by the BBC since Dr Who returned a few years back are all original stories rather than being novelisations of aired episodes. They release six per series, featuring whatever companion is with the Doctor for that season. Or in the case of the year he had no companion, a temporary sidekick for each story. They are not written by the same authors who write the series and if you worry about things like canon then they are not as far as I can tell. I'd classify them as fanfiction, but generally good quality fanfiction.
I bought most of them for the first four series then tailed off and stopped. They can be entertaining, but are fairly short and if you are a quick read one will only last you a couple of days at best. This format was also repeated for Torchwood.