There are walking guides for mainland Greece too, of course. When I visited Kardamili I bought the above book, “Walking in Mani (Book 1)” by Mat Dean. Not sure what has happened to this intended series (obviously?). There is a new book now, “Inside the Mani – a walking guide” by the same author, covering the same ground:
There is an accompanying web page at http://www.insidemani.gr/, but their book shop is dead, so I am unsure if even this one is still in print. Check local book shops. If not available anymore there are several other alternatives, including Sunflower Books and Marengo Publications.