The two which spring to mind are Doctor Who and the Cave Monsters by Malcolm Hulke, which is the novelisation of his own script The Silurians. What makes it stand out for me is the little expansions of what we see on screen. We get a chapter told from the point of view of one of the Silurians, we learn something of life in their time and there are a series of brilliant insights into various characters, minor and major. Things like Miss Dawson worrying about people one day won’t be asking “when don’t you get married?” and instead will ask “Why didn’t you get married?” Later we have a good paragraph or two about a taxi driver and his musing on his fares.
Oh and the hideous goof identifying a T-Rex as a mammal…. oh dear
Also, The Time Machine by HG Wells. It is a short easy read which tells a thumping good story and has some interesting postulations on the end of time.