Although there aren’t actually any textbooks that teach Cantonese to Hong Kong students, there are a lot that teach Standard Chinese.
A few months ago, I got 詞彙寶庫及寫作練習 for 小一, published by Pearson. Unlike TV shows or manga, it’s designed to teach you to improve your Chinese reading and writing skills – and of course, it’s all in Chinese. A possible downside is that it might not be as interesting as TV or manga, but I’ve definitely found working through it slowly to be a part of a healthy balanced diet.
What textbooks can you get for your language?
(One of my favourite things about this book is that it includes some exercises that work on ‘grammar’, which is very hard to find in Chinese!)