From Springer Nature's press release announcing an OA book deal with Max Planck:
The initial three-year agreement, live as of 1st January 2022, will enable authors from all 86 Max Planck Institutes to receive a discount on the standard Book Publishing Charge (BPC) to publish their book OA. MPDL will contribute central funding toward the coverage of the discounted BPC, lowering the costs for authors even further. The discount and funding will be available across all of the publisher's book imprints, under a CC BY licence, ensuring their work is freely accessible and discoverable to all communities across science, technology, medicine, the humanities and social sciences. They will be available to readers around the world via Springer Nature's content platform SpringerLink.
Deals like this one are an extension in spirit of the journal-based read-and-publish model—even if there's no straightforward "read" component. Springer Nature charges $15,000 to publish an OA book, via a Book Processing Charge (BPC). Max Planck researchers won't see that charge, as the German institutes will cover the difference after the deal's discount. The problem is that deals like this prop up the BPC system, which excludes the vast majority of the world's scholars. Researchers lucky enough to work in wealthy European countries and a handful of rich North American universities, meanwhile, accrue all the OA benefits.