You mean, their officially
non-canon books.
And I ask this question ever time someone says
"non-canon books": "Says Who"?
The authors themselves call their work "soft canon" and they will tell you that they have been contracted by the owners of the franchise to continue the storyline. What's non canon about it?
I just had this conversation with somebody. What does "soft canon" mean? It
sort of happened?
What does "canon" mean? It happened? Erm, no, it didn't. It's all make-believe.
Canon is just something people have to keep in mind when writing for the franchise (either for a TV show, film, novel/novella/short story, comic or game). It determines what they are or aren't allowed to do in the story.
Canon means that every show, film, book, comic, game... in the franchise will have to treat it as something that 'happened' (even if it creatively reinterprets it, when circumstances allow it, as
The Good That Men Do did with TATV).
Soft canon or
secondary canon, would, I presume, mean that every official Trek book from now on (with a few exceptions like the books related to the Star Trek Online game, which has it own continuity) will have to treat it as something that 'happened'. Any future shows and films could, however, ignore it... but since there are extremely unlikely to be in more films or episodes continuing the stories of ENT (or TNG, DS9 and VOY), this is a moot point. This is different from the earlier standalone Trek novels from before 1999, which had no bearing on any later literature and could be contradicted any time (and since they were released during the shows, were often soon contradicted by the shows themselves).
In other words, whatever happened in the TV show ENT is binding for the writers of any ENT book, comic or game. But whatever happened in the official ENT books is binding for writers of any future official Trek literature. If, say, new writers were to take over from Martin and Mangels and decide to take the story in a different direction, they would still have to acknowledge everything that happened in the previous books and incorporate it in their own stories, and continue their stories from what happened in 'The Good That Men Do', "Kobayashi Maru' and 'Beneath the Raptor's Wing'.
Now, fanfiction has its own rules and doesn't have to abide by any official continuity. Obviously, a fanfic writer doesn't have to include the books in his/hers own personal 'canon' - but some fanfic authors also often decide to ignore some elements of the official canon of shows and films. Others choose to creatively reinterpret some things that they don't like in canon.
Which is OK - we all know that there is lots of nonsense in films & shows just as there is in books... Official "canon", after all, includes
Spock's Brain, Star Trek V: The Final Frontier, Code of Honor, Angel One, Up the Long Ladder, Fascination, Profit and Lace, Threshold, These Are the Voyages...