22) Doctor Who - The Highlanders, by Gerry Davis
The best Davis novel I've read so far, though this is not especially high praise. It's one of Polly's better stories, and of course introduces Jamie as a regular; a couple of odd changes of detail from the TV version, but this is basically a narrative that hangs together on its own merits. Unfortunately there is still something of a sense of the author writing down what appears on the screen.
23) Doctor Who - The Underwater Menace, by Nigel Robinson
This is very poor. It's not quite as bad as Robinson's novelisation of The Sensorites, and in the earlier chapters I thought it seemed quite promising. But the prose soon descends into his trademark clunkiness, and the story's most famous line actually manages to come over even worse on the printed page than it does in the original.
24) Doctor Who and the Cybermen, by Gerry Davis
A relatively early novelisation here, but not an especially good one. Davis' characterisation is poor (Jamie is thick; Polly is a girlie; the head of the Moonbase is from Yorkshire) and the science of the story still makes no sense. Davis' style must have improved over the years - this and Doctor Who and the Tenth Planet are markedly inferior to Doctor Who - The Highlanders.
25) Doctor Who - The Macra Terror, by Ian Stuart Black
I enjoyed this more than I had expected to, chiefly because of Black's characterisation of the Doctor, which seems to me to capture Troughton's performance better than any of the novels I have read so far. We do, of course, miss out on the superb soundscape of the original (alas, the video is no longer available), and poor Polly ends up screaming a lot. But it's a worthy attempt.
26) Doctor Who - The Faceless Ones, by Terrance Dicks
Another valiant effort here - Dicks actually makes a decent fist of a confusing and incoherent story, featuring the quiet removal of Ben and Polly, and also one of the greatest companions-who-never-was, Samantha Briggs. Dicks has Jamie somewhat intimidated by Samantha's sexuality, which contrasts with what I remember from the original version. He also introduces the sonic screwdriver several stories early, and yet again finishes by promising that the next adventure will be better (though in fairness, each time he does this he is right). But in contrast with even some of Dicks' own less inspiring efforts, it's not bad.
In summary, your life will not be incomplete for lack of having read any of these! These are the five books featuring Ben, Polly and Jamie in the regular cast; it is remarkable how much more interesting Polly is as a character than the other two. Shame she didn't stay longer.