I suspect most readers of TV tie-in books will remember reading as a youngter at least one or two of the Whitman series of original television adventure hardback books, based on the popular TV shows of the time.
Whitman was associated with the Western Publishing Company which published the Dell and Gold Key comics, which between them produced dozens of TV tie-in authorized, adventures. My favourites were their science fiction and adventures comics and books based on my favourite TV series.
The books were unsurprisingly, aimed at young adults, and the plots and characters didn't necessarily reflect what we saw on screen, which makes me think the authors hadn't seen the shows, or possibly had only general synopsis to work with.
While I have a fair number of Gold Key comics in the collection, I only have three original Whitman TV tie-in Authorized Adventure books.
Voyage to the Bottom of the Sea;
The Green Hornet;
Land of the Giants.
I did have the two Man from UNCLE books, and the Star Trek tie-in books, but they went AWOL during a house move.
Lucky for me the Star Trek: Mission to Horatius edition was reprinted by Pocket Books in 1996 to coincide with the 30th anniversary of Star Trek.
Here's a few photos, along with their Gold Key contemporaries.