We don't follow the extra back story in the companions mainly because we haven't read them all, the writers of those started making up their own ideas, even contradicted themselves and they basically came out with a new one every year.
Actually for the most part, each new one just added an additional game (a new chapter). Most of the chapters themselves remained the same.
The only minor changes that occured were to the introduction and World of Daventry chapters. The latter was just expanded each time.
The author, that's Peter Spear for the most part (except for KQ6 chapter, and co-writing the KQ7 chapter with his brother Jeremy Spear), added in his way out of the occasional continuity snafus by stating the world is "always reinventing itself", and is in "magical flux" changing daily. Actually most of the "continuity errors" that exist in the books also existed in the games themselves; I.E. Geography of Daventry (or the architecture of Castle Daventry) changing in almost every game in the series (among other differences).
There are also a few details that may differ from one's interpretation of the game, because there are proper solutions that all add up to the same point solutions or outcome. For example Graham takes the Nightingale from Hagatha while she is out of the cave (whereas equally possible he could have taken it from under her own nose), both outcomes are possible KQ2, and will result in the same number of points.
In KQ6, there are actually several ways to learn about the Island (first talking to the Guard Dogs, first talking to Ali, or first talking to the Pawn Shoppe owner), as well two ways of getting a "maiden's hair", and different order of sending gifts to Cassima, or when the gifts are given (I think its possible to actually give Cassima the ring through the wall if you didn't give it to her early on). Each of these choices can change certain elements of the game's story, and in some cases lead to minor differences in the game ending sequence (yet still offer full score). Thus each player's experience in the game may vary with another player's experiences (so there isn't one consistent solution/storyline).
Another good example is that there are multiple solutions (by a choice of items) in KQ7 in a certain situations that will both result in similar outcomes, I.E. the desert spirit will offer you the choice between two seperate items from his remains. Both can be used to defeat the scorpion but in completely different ways. The novelization chooses one of those two methods, but a player might have taken another route.
There is also non-linear aspect to many of the game's storylines (based on the order player's may have solved the puzzles each time they play the games). So the order of when puzzles are solved may vary between players, and accounts. For example the moment when Graham/player saves the nightingale could occur before and even after talking to the Little Old Lady of the Antique Store.
Will any of these instances where there are two or more equal solutions, where you as a team would have had to pick one story route over the others, have any affect on your story (or other people's interpretations of the stories)? Who knows.
There are a couple of places where the books differ from the games, but that seems to be more because the authors were working with early concept storylines of the games. The authors actually even have an internal in-universe explanation for why there might differences between what is shown in their book, and what is shown in the game (back to the magical flux, and the world being controlled by dreams). There were also a couple of bits in the An Encyclopedia of Daventry section that don't exact fit as later material was released, but most those were written as "speculation" anyways, and the author made sure to state that they were just rumors or speculation on part of people in the world or his own. This actually added more authentic aspect to the story in much the same way that historians and people on earth don't always agree with all the finer details and have alternative theories for what actually happened in earth's own history. Thus take one topic for example, and you'll find that two descriptions from two authors of the same events don't always match up exactly (as they both have their own interpretation of the events).
As a stand alone product the most of the editions are pretty consistent with each other (and most of the inconsistencies are explained away within each additional edition).
