At the end of 2006, the Hong Kong Tourism Board published a booklet I helped with (writing, and contributed some photos).
Discover Hong Kong Nature: 84 pages, covering range of places including the Peak, Mai Po Marshes, Tai Long Wan (Sai Kung), Cheung Chau, Peng Chau and Tai O. English only (so far). Shot here shows cover; and some pages inside.

Free (aimed at overseas tourists) from HKTB places - including in Causeway Bay MTR station (exit towards Jardine's Bazaar), also Star Ferry TST pier.
Or you can download pdf files with the info, maps and photos via the HKTB page
Discover Hong Kong Nature (though downloads didn't work for me with Firefox; on my Mac, ok with Safari).