This kind of reminds me of Arron Conner's and the Tex Murphy series.
The first two games are both novels too. Maybe it was something the creators did at that particular time with 'cult' point-click adventure gaming.
I will take a look at Gabriel knight 1 as I don't want to know the plot to number 2 till i have completed it, when released on GOG. :)

I own the first book and would love to read the second. The first book doesn't really add any insight to the story, though it does add some internal dialogue to round out Gabriel's persona. For the most part it's just a straight novelization of the game and it doesn't really stand on its own as a real novel. Enjoyable to read, however.
The only thing that annoyed be about the book was that Jane took out the "Oh bless us, Wilbur" line from Gabriel's impromptu blessing.
In GK3 at the beginning of the game there's a bookstore with the first novel in the window!

The last book I read that was based on a game was a horrible experience. It was so bad (Resistance: The Gathering Storm), that I swore off game novelizations forever! So unless this series is the cat's meow, I think I'll pass and just play the games...

Beast Within actually adds quite a lot to the storyline, and includes the deleted chapter where (in the game) you were controlling Ludwig. You could consider it a 'Director's Cut' of sorts.

The nice thing about the GK books is that they're written by the same author as the games themselves. No need to worry about straying from the creator's vision.

One significant difference in the novelisation of GK1 is the scene where you have to break into Mosely's office. It's entirely different in the book, and very creepy.

I've read both books years ago and found them to be quite good.
I even think that both books can be enjoyed by someone who hasn't played the games - perhaps even more so as especially book 1 adds almost nothing new to the story.
Jane Jenses is a brilliant author and I strongly suggest that you read something she wrote that has nothing to do with Gabriel Knight.

I do own the Beast Within novel, took me forever to track it down in a not completely beat up condition. Used books can be wonderful when the original owner cared for their books. :P I've pondered also getting the Sins of the Fathers one, haven't heard as much praise for it as I have for Beast Within but I love Jane Jensen's writing, so I'd rather decide for myself if it's worth it.