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.
It was 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 could download pdf files with the info, maps and photos via the HKTB page Discover Hong Kong Nature, which – by summer 2012 – has sadly been abandoned.