Here's the thing: no tie-in book, in any franchise, is ever truly "canon" (in the sense some readers want it to be) because it can and will be contradicted by the actual movies or tv series as they see fit. Because nobody making a multi-million dollar movie is going to worry about whether it contradicts an old paperback novel that only 2% of the viewing audiences have ever heard of.
Also because canons even contradict themselves when it suits them. Continuity is not the exclusive goal of fiction, it's just one of the ingredients in creating the illusion, and sometimes that means a canon changes itself and just pretends it was that way all along.
Most of the audience doesn't fixate on small details. Heck, there are plenty of viewers out there who don't understand that the different screen interpretations of Batman are out of continuity with each other. There are some who don't even realize that Batman and Iron Man inhabit different universes. They just watch the stories and don't dwell on the minutiae.
And, honestly, speaking as somebody who has been editing and writing tie-ins for most of my adult life, I can testify that I have never seen the word "canon" appear in any contract or licensing agreement, nor does it often come up with working with a licensor.
Trust me, if I'm negotiating to acquire the rights to publish SPACE MERMAID novels -- based on the hit TV series -- we're going to be talking advances, royalties, territories, schedules, image approvals, and so on. The burning question of whether the books are going to be "canon" is not going to be discussed.
In the the real world, and in any practical sense, it's a non-issue.
Right, because any tie-in is going to do its best to stay consistent with the screen series it's tying into. That's a given. Whether it's canonical is a matter of whether the makers of the screen franchise choose to acknowledge it in return, and that's not under the licensees' control. Our job as contracted tie-in authors or editors is to follow the franchise's lead. They're not obligated to follow ours. Unless they choose to, but they very rarely do, and only to the extent that it suits them.
I will also note that I found the quality of the SW tie-in novels to be vastly inferior to that of the ST tie-in novels of that era (and canon has precisely nothing to do with it; the books either bored me, or grossed me out, or in some cases both). Compared to ADF's Splinter, I found the Han Solo and Lando Calrissian trilogies to be disappointing (and I eventually sold them off to used book dealers). And I wasn't all that happy with the quality when the imprint changed from Ballantine/Del Rey to Bantam Spectra, either. And I sold off most of them, as well. I'm pretty sure that the only SW novels I still have are either movie novelizations, or by ADF, or both.
Again, keep in mind that the early tie-ins like
Splinter, the Han and Lando trilogies, and the Marvel comic were a separate era of SW publishing than the Expanded Universe era that began in 1991, despite the EU's efforts to force them into the continuity retroactively. Back then, there was no overall plan, no attempt to connect the tie-ins in a shared continuity. Each one basically did its own thing.
And there's an exception even to that: it seems that Gary Wolf explicitly declared his own Who Censored Roger Rabbit to be non-canonical (as I recall, he declared it to be Roger's nightmare), because he liked Disney's version better, and started writing in the Disney version of the milieu.
Or maybe because he recognized it was more profitable to write sequels that audiences would recognize as compatible with the Disney version. The writers of
Logan's Run did something similar, starting the second novel with a brief passage that undid the original novel's ending and changed the situation to something more akin to the completely different ending of the feature film. And Arthur C. Clarke's
2010 novel was a sequel to the film version of
2001 (where the Monolith was at Jupiter) rather than the book version (where it was at Saturn).
