I know Smallville has two series of books, one young adult and the other more general, for a somewhat older audience.
They did the same with the Star Trek
novels. The BtVS novels, too. Some of the BtVS novels were written specifically for "young adults," and tended to be shorter and...well, less mature than the "adult" novels.
They didn't do this for the Angel
novels, as far as I recall. Understandable since BtVS, which was set in high school for a large part of it's run, is much more easily...teen-ified.
I've read more tie-in novels than I can count. And they are like anything else. Some are good and well-written. Others, not so much. I've read tie-in novels that felt like stepping into the show. I've read others which, aside from being set in the same place, and having characters with the same names, felt nothing
like the show.
Basically, it's not being tie-ins that makes some of these novels bad. Some tie-in writers, even with all the restrictions, manage to turn out some great stories.
As for the restrictions, I think it differs by fandom. I think whether the show is still in production or not might also be a factor.
Edited by Bitterswete, Jun 5, 2007 @ 1:17 PM.