I think the "digital folder" part is the loser here. It sounds like the difference between grabbing a collection based on the client and project (or date, or...) with the RedBooks, vs. reading a book via its index (digital folder). That is, the UI for a RedBook vs the UI for accessing a digital folder are fundamentally different, and the digital folder hierarchy is/was demonstrably worse. Sounds like a banal conclusion I'm sure, but the organization of these kinds of assets can actually be done wrong.
It comes down to the curation. Those books were the result of one or a few people consciously deciding what was important to include. They made value-based decisions to decide what would make the cut. Constraint forces choices which lead to a more valuable experience.
In addition to the physicality of the book, browsing them meant the viewer didn't need to sort through the cruft (someone had already done that work for them), allowing them to use their own energy/thoughts to build upon that work resulting in new ideas and creativity.
