(OP here) I discussed how the DRM works in an earlier blog post - https://shkspr.mobi/blog/2025/03/some-thoughts-on-lcp-ebook-...

Essentially, the key for decrypting the files is made up of the book owner's passphrase and the super-secret key embedded in the closed-source binary.

I wasn't able to reverse engineer the binary or extract that key.