Repeat after me: No tie-in book is ever canon, no matter what you may hear, for the simple reason that the movies and TV shows are viewed by millions while the books are read by the tens of thousands. No studio in their right mind is ever going to let the tie-in books, which are read by only a tiny percentage of viewers, establish "canon." The actual shows and movies trump the books, period.
It's not an artistic decision. It's the only practical one.
Eh...I can think of one major exception to tie-in books not determining canon, though it is outside the specific "movies and TV shows" category you mention: Warcraft. In fact, it's a not-uncommon complaint among the player base that major revelations are only presented in novel form and only alluded to in-game, but still binding on the storyline in-game. (And being an MMO, and as such having little player choice influence on the plot, it legitimately is a situation whereby major story elements in a non-interactive storyline are described entirely in a licensed side medium taken in by only a fraction of a percentage of the fanbase.)
For one specific huge example, the ultimate fate of the main antagonist of the previous expansion was only described in a novel, and then that fate was directly influential on the current expansion without ever actually being shown in-game, only referenced. In another case, two expansions ago a major character that had been missing for years as a huge plot element that had been hinted at for most of that time as a dangling plot thread reappeared with little fanfare in the in-game narrative seemingly out of nowhere to those that only played the games, because the story of his reappearance was laid out in a novel. That same plotline also involved a separate major character present from the start of the MMO suddenly becoming an antagonist, and again those that only followed the storyline in-game would have no understanding in it beyond a vague description about how it happened.