Well keep in mind the KQ novels are more tied to KQ marketing department than anything to do with Roberta. She only read them as far as I know.
They are Kingdom of Sorrow and See No Weevil are set between KQ2 and KQ3 (roughly 7 years and 15 years after KQ2). The period is portrayed as mostly peaceful, although there are a few references to Alexander being missing. So depending on how much a purist you are you might think of them as contradicting the concept implied in KQ3 that suggests that the Dragon should have already been destroying Daventry only a few years after Alexander was kidnapped, and Graham sending maidens to be sacrificed for years. The two novels never mention the dragon, so it would imply that the dragon returned shortly before Alexander returned (at least it would have had to have appeared shortly after See No Weevil), giving only enough time for maybe 3-4 maidens (including Rosella to be sacrificed).
The first novel was The Floating Castle, which takes place roughly after KQ4 and probably before KQ5, although the epilogue may have to be after KQ5 due to the amount of time covered in the novel. Although Alexander is shown to be in good spirits, and not remorseful soul that he appeared to be between KQ5 and KQ6 after meeting Cassima.
The cool thing is each book spends at least a chapter in the castle, other chapters spend quite a bit of time describing the kingdom itself. The town/village of Daventry briefly appears in Kingdom of Sorrow. See No Weevil spends quite a bit of time in the castle, and nearby town of Daventry, and countryside of Daventry. There is alot of political bits on and everyday life in the kingdom, that obviously the games never cover.
Between all three novels while there are a few descrepencies on how parts of the castle are described, although in general they all describe different parts of the castle, giving a bigger idea of what it is like inside the castle. In general the descriptions of the Great Hall, and halls leading to it, and courtyard, are a combination of KQ1 AGI version of the castle, and KQ1 remake of the castle. Floating Castle is probably more like the remake, and the other two describe more like the original. The moat monsters from the remake get a nod in SNW, and KoS.
Kingdom of Sorrow in particular gets quite a bit out of the Kingdom of Daventry proper to a nearby lands.
All three also link together through a few encounters with the inhabitants of a nearby kingdom on the border of Daventry, called the Old Woods, ruled by fairies.
Then there is the King's Quest Companion, that has a novel for each game up to KQ7 depending on the edition you have. The novels are written in such a way that they can function as a walkthrough for each game (so even something that may seem to be more of a 'game mechanic' is described in the novel so that the player if they are using the novel to play the game will still understand how the game functions). Each novel is written from a different perspective. KQ1 for example is written by an unnamed court chronicaler, same one who wrote KQ1 prologue story (it is essentially a novelization of the original KQ1AGI). KQ2 is written by Gervain (the prime minister mentioned in the KQ2 manual prologue story), KQ3 is written as an interview between Derek Karlavaegen and Alexander-Gwydion, KQ4 is written by Valanice, KQ5 is written by Derek Karlavaegen, KQ6 is written by Derek Karlavaegen, KQ7 is written by an unknown author from the world of Daventry (concerns Valanice and Rosella telling King Graham about their adventures in the world of Eldritch).
While the actual author of most of these chapters is Peter Spear, the author of the KQ6 novel was a professional fantasy author; pen name "eluki bes shahar" aka Rosemary Edghill, and the KQ7 material was co-written with Jeremy Spear (pulled from their KQ7: The Authorized Guide).
In addition there are a handful of other chapters that written as letters, essays or articles from Derek Karlavaegen, Alexander or others that offer more insight into the world of Daventry, on topics such as magic, geography, customs, etc.
The first two editions had an additional chapter 'An Encyclopedia of Daventry' that went topic by topic from Abominable Snowmen to Zombies giving more insight into various characters, creatures, places etc, found in the first five games. As well as information pointing out the myth or fairy tale inspiration behind many things found in the KQ world. In addition the first three editions had three sets of maps, showing the world as the series progressed. As well as two sets of artwork, portraying an event related to each game.
The special thing about the King's Quest Companion is that the main editor, and author for 90% of the book, Peter Spear was a friend of the Williams, and others at Sierra. He often worked directly with Roberta or other designers on the KQ games in writing the ideas for his book, and he was able to get inside access to games while they were being developed (although this left a few discrepencies if ideas were changed in the finished product, see KQ6 novelization for a few examples).
http://kingsquest.wikia.com/wiki/The_King%27s_Quest_Companion [url=http://kingsquest.wikia.com/wiki/Romancing_the_Throne:_From_the_Chronicles_of_Daventry,_Part_II]http://kingsquest.wikia.com/wiki/Romancing_the_Throne:_From_the_Chronicles_of_Daventry,_Part_II[/url]
http://kingsquest.wikia.com/wiki/Prince_Alexander's_Own_Story!_Exclusive_Interview
