If I understand correctly, you're looking for a way to express that your execute business-layer function needs a Book-like object without actually using the Book class. If that's not the case, I apologize.

One option is to guard the import in add_book.py with an if typing.TYPE_CHECKING clause. This would mean that it's not imported at runtime, but you can still check the code statically with mypy/pyright or in your IDE. If that's still too much dependence, and you're not looking to import the object at all, then you'd probably want to define a Protocol to specify the interface that your function expects—

from typing import Protocol

class BookProto(Protocol):
  isbn: str

def execute(payload: BookProto):
  ...

This can be cumbersome if you have a lot of models or functions, but it's probably the safest way to express that you want "something that has an isbn: str attribute" without requiring the rest of the model. This would allow you to pass the Book model in your route, but (probably, I haven't tried this) also allow a dataclass with isbn: str if you strip out pydantic entirely.