If you only choose books/manga with furigana the number of kanji doesn’t matter.
Also, maybe it’s not in the right place, but the Master List of Book Clubs mentions this club being N5, Beginner Book Club being N4, and Intermediate Book Club being N3. But as @Abstormal said, it’s really hard to define more specifically.
That’s what averages are for… If a book has a 2.5/5 and another has 3.5/5, you should be able to say with some confidence that the first is easier.