Anakin I think this discussion was already made, this may need to be merged...
As for the books, I found them entertaining, have read them a few times... They aren't 'excellent', but aren't 'horrible' either! Maybe there is a little 'meh'. Especially with the See No Weevil.
I think the best bits, was they really go about describing the kingdom and the castle... More than any other source... and the politics of the kingdom!
Personally I liked all three a lot and wish that each game had had a full novelization of it--like not just in the companion, but released as separate books
Keep in mind that the Companion was primary designed as a game walkthrough/hintbook, not a novel. It takes the games pretty literally, explaining everything from a way that a player would be able to solve the game. Including explaining some of the gameplay quirks like timer in KQ3 (or utilizing the wrap-around in the early games). Everything a player would need to know to avoid death and beat the game, as quickly as possible, and avoid too much backtracking.
The author/s took a rather interesting approach to writing a walkthrough, but he could just as easily wrote it as a standard walkthrough.
The games are pretty short, and not really full 'novel' material, unless an author went out of his/her way to completely retell the stories, add characters, make the plot less like a game.
Compare the differences in actual novelizations for games such as the ones written for Gabriel Knight series. The novels are not intended to be walkthroughs for the game, and deviate quite a bit in many places, especially in the second book.
Then there was the author of the Police Quest Casebook, Peter Scisco, that didn't bother to follow the games, and just rewrote the stories to be entirely for the purpose of fiction 'novels'... It's a complete mess, more entertaining as stories I suppose, but seriously detours from the game's events (changes many of the characters, combines characters, removes characters, etc)... You can't use his 'novels' as walkthroughs, since they leave out most of the game's solutions, and even change many of the game's events (often completely different events occur). For example in PQ4, the main character has a sidekick/partner for most of the story (who is murdered near the end), when you as a player work alone in the game! In the novel this partner does many of the actions your player actually does in the game, while the main character is off doing things completely outside of the scope of the game! Another example, is the novelization of PQ2 leaves most of the entire terrorist subplot, including the hidden bomb on the plane... It also has a massive shoot out on the plane, between the terrorists and Sonny Bonds (there is no bomb)! Now, in the game if you try to shoot and aren't careful, you die! If you don't discover the bomb you die before the plane lands! So it completely changed these events for the sake of dramatic purposes.
The book even goes out of its way to point out that the novelizations are not intended to be walkthroughs, but written for 'entertainment', and "ficticious stories based in the actions of the game series". It leaves actual accurate hintbook to seperate chapters between each 'novel'.
Frankly the best comparison would be look at Space Quest I vs. the Comic Book adaptation! The adaptation is not, and not intended to be a walkthrough!