John Chapman has a page with some references:
"Bushwalking Guide Books - Western Australia"
http://www.john.chapman.name/pub-bwa.html (Last updated: October 2010)
There is also a book on the S-W:
"Bushwalks in the South-West"
Bushwalks in the South-West is a compact, practical guide to some of the most scenic walks in the south-west of Western Australia. It contains simple maps and clear descriptions of 48 walks. You can wander through the jarrah, karri and tingle forests, and explore the mountains and sea cliffs from Mandurah to Esperance. The walks can be easy or challenging, and they range from a few hundred metres to 15km.
http://www.exploroz.com/Shop/WACALM+Bus ... West_.aspx