I work with different art styles (as you can see from my galleries), so when a big project comes along - it's important to start looking for resources to help aid the process of creating a fitting art style for such a project. I've found several useful ones, and will probably continue to do so as I work with other artists on [redacted, wink wink].
As always, if I see find more information - I'll be sure to update this post (or create a follow-up).
Websites
Game Art Bible - Secret Sauce To Making Great Game Art (By Kshiraj Telang)
This slideshow makes a useful checklist guiding you to make your desired art bible/style guide. It's clear, simple, and short - and it's my go-to to when I need to step back and think of what needs to be done. Wayback Machine link.
A detailed example of an art bible that sets clear boundaries and leaves room for creativity. Wayback Machine Link.
Game UX Style Guide: Why Do You Need One? (By Om Tandon, UX Director at Wildlife)
A post filled with useful information on what to think about as a UX Designer for a game. I find the numerous terminology used to be relevant and a good starting point to go searching for more information on the same matter. Wayback Machine link.
How To Create A Style Guide From Scratch. Tips and Tricks. (By Andrei Dorin)
A more website-design based post, with useful examples of code. Wayback Machine link.
Creating an Art Bible (By Veerle Zandstra)
This page is a great example of what an art bible can look like as well as being as an inspiration. Wayback Machine link.
Videos
Aesthetic Driven Development: Choosing Your Art Before Making A Game (By Vladimir Slav, ColdwildGames Dev)
A great talk on how ColdwildGames devs experimented with various pixel art styles that would garner attention and a following, as well as finding the right balance between feasibility and aesthetics. Wayback Machine Link.
Developing the Art of Fortnite (By Peter Ellis)
A must watch. Wayback Machine Link.