You don't need to convert it. List<T> implements the IEnumerable<T> interface so it is already an enumerable.
This means that it is perfectly fine to have the following:
public IEnumerable<Book> GetBooks()
{
List<Book> books = FetchEmFromSomewhere();
return books;
}
as well as:
public void ProcessBooks(IEnumerable<Book> books)
{
// do something with those books
}
which could be invoked:
List<Book> books = FetchEmFromSomewhere();
ProcessBooks(books);