I'm a big fan of the Cosy Places subreddit, but quickly learned a key tell that a lovely room isn't lived-in: libraries of uniformly handsome or antique books that tell you you're looking at a set, or the corner of a furniture store.
The reality is that books tend to be a "noisy" presence, designed to advertise themselves and exhibiting loud and trendy design fads. Ready sources of cheap books — library clear-outs and estate sales — only reflect this state of affairs in bulk.
So it turns out there's an entire industry of books hand-picked and organized to look good, sold in bulk according to a variety of visual or conceptual themes. Color gradients is a hot trend in the world of books sold by the linear foot.
Just a few stores I found: Books by the Foot, Zubal Books, The Book Bundler, Decades of Vintage and Booth & Williams all specialize in books sold for their aesthetic appeal rather than their contents.
There are also a number of books-by-the-foot/color bundlers on Etsy, on Amazon and on eBay, but shipping tends to me more expensive.