I don't know about Black Cats? Do your boy like Thomas? I find a lot of Thomas books in discount in this website. u can take a look.www.ngatinggroup.com.hk
Black Cat is a series of English books, there is 5 books in a set (5 audio CDs attached), the publisher is HK Commercial Press. Now is promoting Level 1C which is suitable for P.5 to F.1 students. I bought it earlier and find the books very good, my son likes them much.
Please refer to the following website for details:
Black Cat is a very good series. My son is also listening to this set. If you buy the HK version, it cost around HK$48-$64 per book, depending on the level. You may also consider buying the mainland version in those Mainland Bookstore, some in Mongkok. Hk & mainland version are exactly the same except the explanations of the vocabulary at the bottom of each chapter are in simplified Chinese for mainland verion. mainland verion is ~$20 per book.
Black Cat is a good series. You can borrow the story book and CD from public library. There are about six level (level 1 to 6) for Black Cat series suitable for different student.