As far as pre 2014 books go, the RotS Novelization is about as canon as you can get, As Lucas was so heavily involved in personally editing it. Matthew Stover stated in 2006:
"I received from LFL a Word document of Revenge of the Sith with Mr Lucas' edits, which was distinct from the edits I'd already gotten from Sue Rostoni and Howard Roffman and the rest of the LFL crew, and this document was edited in such a detailed fashion that even individual words had been struck off and his preferred replacements inserted, as well as some passages wholly excised and some dialogue replaced with the dialogue from the screenplay. If that's not line-editing, I don't know what is.
What's in that book is there because Mr. Lucas wanted it to be there. What's not in that book is not there because Mr. Lucas wanted it gone.
Period."
Okay, but Lucas is no longer deciding what counts and what doesn't. The thing canon-preoccupied fans never understand is that canon is not carved in stone -- it's just the way the current creators choose to tell the story. Different creators can ignore things their predecessors considered canonical (like Jeri Taylor's Voyager novels), and the same creators can change their own minds about canon in later years (e.g. Han shooting first).