Novelisations that add to or alter the story
As discussed on another thread (on Ghost Light), some novelisations are simply a "reminder" of what happened on TV, which was great in the days before DVDs, before VHS and before repeats of Doctor Who were common.
I like it when a version of a Doctor Who story - especially an early one - hints at a completely different chronology, or else enriches existing chronology.
Novelisations that change perceptions include:
The Daleks/Doctor Who in an Exciting Adventure with the Daleks. A completely different beginning that deletes An Unearthly Child from continuity (replacing it with a drama on Barnes Common), and changes the character and backstory of the regulars. Susan's surname is English, not Foreman, and she doesn't go to a school. Ian is a scientist, not a teacher, and he has a nicotine addiction. Barbara is a private tutor, not a teacher. The TARDIS (or Tardis without the "the") is an even more exciting place, with coloured pillars, secure rooms, massage showers and so on. We miss out on some scenes because everything is seen from Ian's point of view. The Ship's console does not explode at the end.
The Web Planet/Doctor Who and the Zarbi. The Ship's pilot is called Dr Who. The number of chapters - 6 - is the same as the number of episodes. There are references to past adventures that don't quite fit with what we remember, such as Susan marrying David Cameron (!) and the Ship being magnetised by their encounter with the Daleks. Sadly it's a poorly conceived story, and the writing is not great, and we lose one of the best scenes from the TV serial. Curiously, Ian's recent past as a Coal Hill School teacher has been unretconned, and Barbara (veteran of many adventures on other planets and in France) is surprised that the Menoptra of Vortis appear to speak English.
The Crusade/Doctor Who and the Crusaders. The main plot is much the same, with a few interesting variations. They don't jump a time track at the end. But the most enrichening aspect is the prologue. It's a leisurely moment inside the Ship between adventures. There are references to untelevised past adventures including a trip to a planet of talking stones. Vicki and Barbara play Martian Chess. Ian questions the Doctor about the mutability of time, and the Doctor hopes their next adventure will involve two sides that both think they are in the right - and he gets his wish.
The Chase & The Dalek Master Plan (3 novelisations). I've only read the first of the two Master Plan books, but I understand the three books were restructured to make it a coherent trilogy. So the Marie Celeste is corrected to Mary Celeste, and the Doctor does not break the fourth wall to wish us a merry Christmas.
The Massacre. The novelisation belongs to a different continuity - and is all the better for it. There is a silly bit in which the TARDIS is burnt at the stake (or not), but it's worth getting the book (or the unabridged audio reading from Audible) to get a completely different - and thoroughly intriguing - take on the familiar story. The most interesting part is the Hartnell Doctor meeting the Time Lords in a garden, and speaking to them in French.
The Tenth Planet. In the TV serial, this story immediately follows on from The Smugglers, with a rather odd moment in which the TARDIS interior is unable to stay warm when they arrive in Antarctica. In the novelisation, IIRC, there have been several largely uneventful landings on planets prior to their landing in Antarctica, which means Ben and Polly should be seasoned travellers by now.
The Moonbase/Doctor Who and the Cybermen. The first Cybermen novelisation, and one of the only Doctor Who adventures in which gravitation is a thing. At the time it was a little surprising for an 11 year old such as me for a companion to say, "Doctor, I think it was a Cyberman!" I knew about them - as a very young child I had memories of being terrified of the "Simon men" in the London sewers - but in book terms, they hadn't been introduced yet! Of more interest was their back story, which frankly hasn't been beaten - neither in the Big Finish play Spare Parts nor in the New Who serial Rise of the Cybermen.
Colony in Space/Doctor Who and the Doomsday Weapon. Jo Grant's fourth story - or is it her first?
Spearhead from Space/Doctor Who and the Auton Invasion. Jo Grant's other first adventure. [ETA I confused Spearhead from Space with Terror of the Autons.]
Other novelisations I intend to eventually read include The Romans (a series of letters by Ian to Coal Hill School for some reason), The Myth Makers (from Homer's point of view), An Unearthly Child (already retconned out by Doctor Who in an Exciting Adventure with the Daleks), The Edge of Destruction (novelised twice, apparently), The Mind Robber (reimagined not to follow on from The Dominators), Ghost Light and The Curse of Fenric (adds to what we saw on screen).
So this is permanence, love's shattered pride.
What once was innocence, turned on its side.