Same here in a Macbook pro 2019 Intel I9 based updated to MacOS Sequoia 15.2.
It seems the problem is in the import utility, as I can import pdfs and eBooks from my iphone, and then the Books.app shows them correctly (my current workaround: sending everything I want to import to iCloud/Downloads folder, and then open them from files into the Books app on the iPhone).
However, neither dragging pdfs from finder to books, or using "import" from the books app, or sharing a pdf with the books.app works on the macbook.
The problems on my side started after upgrading to Sequoia 15.2 from Sequoia 15.1.x (don't remember which). No idea how to sort this out and eagering for a solution.
Best.