You have a built in market with these books. Fans are going to buy them and read them. Do not piss them off.
I've been burned by these books before. It's years later and I can't even think of Ben Mezrich's X-Files novel Skin without cringing. Having Scully's cross the wrong color was just the beginning of what was wrong with that book.
But that's not what I'm here to rant about today. Today, I am here to rant about the Supernatural novels. Oh how I wish Carver Edlund were real. He would not make such stupid mistakes.
I read Witch's Canyon first and it was pretty good. Interesting plot, Sam and Dean were in character, no glaring mistakes. Then I read Nevermore. Big mistake.
Interesting plot with the Poe stuff, but not executed to potential. Really stupid original characters. A few other minor things, such as desribing Dean as short. The word you want is shorter, not short. Dean Winchester is not short. It's just that Sam is so tall.
But what really threw me were that the eye colors were wrong. Sam had brown eyes and Dean had blue eyes. Wrong, wrong, wrong. Sam's eyes are hazel with more green than brown and Dean's eyes are very definitely green. Another thing is that Dean has never called his brother 'Sammich'. Sam, Sammy, and one time Sasquatch, but never Sammich.
They should have stuck with the guy that did Witch's Canyon. He was much better.