Novelizations and (less commonly) comic-adaptations of films filled this role generally, before home video releases were common & affordable. The Star Wars novelization, for example, didn't sell well because it was a great book, but because most kids couldn't re-watch the movie every single day like they could when the VHS releases came out, so they'd instead read a $1.25 mass-market paperback until it fell apart. They were chiefly memory-refreshers for the actual film. There was even a brief span of time in which audio versions (often of the film itself, not necessarily of the novelization, so, not exactly an "audio book") helped filled this role for some films, because audio cassettes & stereos/walkmans were more widely available & affordable than video decks.

Maybe I'm imagining it, but I wanna say there were things like re-cap books for seasons of shows like The X-Files, even, because home-video copies of entire shows were prohibitively expensive for all but the craziest fans. That's also part of what coffee-table type books about shows & movies were for—lore & encyclopedic content mixed with stills and what were effectively plot and episode summaries.

I loved the Dr Who novelisations, like you say they were the only way to experience the show except through the odd repeat. In many ways the books were better than the actual episodes, unconstrained by budget and time. I have been disappointed by many stories of Dr Who that I originally experienced as a child in book-form, what was filmed just doesn’t compare to my imagination.

In at least one case - "Warrior's Gate", the final Romana story, there was a "2001"-style vibe between the show and the book - i.e. both very good, but it was a lot easier to tell what was going on having read the book!

That was certainly one of the books I was specifically thinking about. From memory, the book starts somewhat before the show and explains the setup better.

The best of the books reworked the plots significantly, even when they were written by the original screenwriter. Sometimes what works on the screen doesn't fit the page and vice versa.

My parents were not movie people- never went to the theater, rarely rented movies. So my introduction to Star Wars was the Expanded Universe novels at the local library.