Hi KL,
If you want to buy really cheap books, you may go to 地鐵 中山站 地下書店街. There are a few book shops. Cite is one of those. Last time, I can find a few 三采漫畫, really cheap. Less than HK$50 per book.
誠品is a bit expensive. However, choices are more and more comfortable there.
I bought books from 博客來 last time. You may pick up the books from 7-11. The service is very OK ga.
I have also bought from 金 "ding". The books were dispatched to my hotel. You may ask the hotel to pass the money to the delivery man. However, my personal experience is the books from 博客來 are of better quality than from 金 "ding".
Have to go to 度小月 and 景泰豐 for meals. I like the former restaurant more.
A good place suitable for Kids: 木柵動物園&貓空纜車.
Some more.... Let me recall my memory and get back to you later.