On tie-in books in general
I don't remember the first Star Trek episode I ever saw. I do remember the first Star Trek book I ever read: Mission to Horatius by Mack Reynolds. The second was Star Trek 3 by James Blish.
I read two tie-in novels about The Invaders in the early 1970s. I didn't actually see an episode of the TV series until the first two episodes were released on VHS to cash in on X-Files mania.
I had seven of eight Tom Corbett Space Cadet books before I realized they were based on a TV series. I didn't see any episodes of the show until the early 1990s.
I've read books based on Get Smart, Rat Patrol, Mission: Impossible, Man From UNCLE, Hawaii Five-O, and yes, Bewitched.
I read the original edition of the novelization of Star Wars. The one with the Ralph MacQuarrie cover released six months or so before the movie came out. I read it as soon as I could find a copy, well before the movie premiered.
Looking at the bookcase to my right, I've got books based on dozens of SF and fantasy TV series and movies. It's not because that's all I read, by any stretch. No, there are two reasons. First, when I started watching SF the only way to have your own preserved version of a show or movie was to buy tie-in books. Second, much more important, and still relevant: a good tie-in book makes the experience of the TV series or movie deeper, richer, and more rewarding. TV and movies can be immersive, but they can also be watched with a fraction of your attention while you eat supper, or IMDB that actor you can't quite place, or make out with your significant other.
Reading demands your attention. You have to immerse yourself more deeply. You can't turn the pages while doing something else and take for granted that you saw enough out of the corner of your eye to keep you from losing track of what's going on. Plus, books aren't on TV for free, and they aren't usually over in an hour. You invest more time and money in reading.
Writers reward that effort and that immersion by going where filmed media can't -- inside the characters' thoughts; in places that aren't practical for filming; they can deceive us and keep us in suspense in different ways from film. Good tie-in writers are giving us a different experience, one we could only have this way, and one that lets us go back to the filmed original with a new perspective on the characters, the setting, the stories.
Sure, sometimes what you get instead is a hack job by someone who doesn't seem to have ever seen the property in question, who gets the basics wrong, who does things that are simply ridiculous. It happens. But it happens in what some folks call canon, too -- look at all the character abuse in Star Trek V, for a start.
If I really like a TV series or movie and I think there's more stories there than we're getting, if it's something that lends itself to expansion in print form, then I'm interested. Sometimes I'm disappointed, as I was by the failure of shows like Farscape, The X-Files, and Babylon 5 to produce strong and lasting tie-in lines. But sometimes the books exceed my expectations. There are whole universes of storytelling in series like Star Trek, Doctor Who, and (though I'm not so much of a fan) Star Wars in book form that go far beyond the filmed canon. Take Vanguard, for example. Bernice Summerfield. And so much more.
Now, if you'll excuse me, I'm in the middle of a Torchwood novel that fills in some of what Gwen and Rhys were doing between Children of Earth and Miracle Day.