While I am promoting other people's books, I could not miss out Martin Williams' great contribution...i'm sure you have read articles of his in the SCMP about nature and our wildlife; He also runs http://www.hkoutdoors.com/ and is a great guide, particularly for birding. Anyway, he has a number of books, but his most recent is one of my faves:
And for the intrepid and also the not so serious hikers, there are these books from Pete Spurrier.
Enjoy your walks and let me have your feedback on these tomes.