I'm not sure. I've not read that many rule books, and when I do, I'm usually either browsing for ideas or I'm looking up something specific, which means I'm not focusing on the quality of the writing - unless it's so poor as to be a problem. And that old excuse I keep using this month: most of my books are packed away so I can't even nose through for comparison now.
I haven't read that many tie-in novels either, one for ShadowRun so many years ago all I remember is one of the secondary characters had cheap cyberware boosting his speed and it made him twitch and stutter, and then most of the Ravenloft books - some of which are terrible, to be fair. Most either have good writing but weak concepts, or interesting concepts and weak writing - but the Strahd novels by P N Elrod are very good, in concept, plot and writing. I'd like to read more by her!
RPGaDAY was started by Dave Chapman and is currently curated by RPG Brigade. To join in yourself, follow the questions in the graphic and blog, vlog, tweet, or otherwise share your responses with the hashtag RPGaDAY2017.