The three original licensed novels by Sierra and Berkley Boulevard in the King's Quest universe.
There is some debate on how closely tied to Sierra these novels are. While they were licensed by Sierra, it does not appear that Roberta Williams nor anyone associated with King's Quest direct development was involved with the books (though Roberta did donate a copy of the first book along with her other works to the Museum of Play).
While some elements appear to be a mix of the original King's Quest games 1-3 and KQ1 remake, there are some inconsistencies with earlier material including (including a difference in scale of the world and Graham's backstory), see Canon and BB universe, which some fans may not consider these 'canon', but otherwise they offer some only imaginative glimpse into everday life inside of Daventry and the castle, that otherwise is overlooked, during periods that are fairly unknown.
This wiki considers some of the differing details from these novels to be more or less an alternate universe to the original King's Quest universe where they contradict each other, but otherwise a spinoff of the original canon (classic Sierra era, as opposed to anything to do with the new Reboot Canon).
As these are neither hintbooks, strategy guides or documentation material they were somewhat secondary to other published materials as well (including the manuals, Sierra's Hint books, The King's Quest Companion, and The Official Book of King's Quest series) which had more influence by actual Sierra employees (though as in the case of manuals often came from the marketing and documentation department with little direct input from the developers, with a few exceptions)...
Some reinterpretation of the material with regards to a more cohesive continuity with other canon material is discussed in most articles. Overall these books are treated as canon or at least a secondary canon to the main canon of the original series as that is what they are most inspired by.