As for me this is very simple kata (when I read requirements for each kyu I choosed 4 kyu). And this is very-very interestingkata. And I want intermediate version of kata (with allmost all support of SQL Select version). And I want advanced version of this Kata with support of INSERT/UPDATE and table indexes (so code that doesn't uses indexes will got timeout).

Thanks for kata!

You pass into my function a list of categories. And there is a requirement to return collection with category->count (event if count is zero). I must add a special condition to meet with requirement 'when books are empty return empty collection'.
So I suggest you to remove this requirement. If list of books is empty then we must return collection with all specified categories (and count = 0)

It's very bad that you require that: if books or categories are empty return empty collection. Please, remove this requirement