New Post: The source IQueryable doesn't implement IDbAsyncEnumerable

You will need the IQueryable from EF before SQL plan has executed.

As documented here: https://genericunitofworkandrepositories.codeplex.com/wikipage?title=Adding%20Custom%20Queries%20to%20Repository&referringTitle=Documentation.

You need to access the IQueryable for Book as follows:

var books = repository.GetRepository<Book>().Queryable();