I don't know of a guide book specifically for the island Chris. Publications like the Rough Guide include Symi but it's usually covered in just a few pages.
The best map is by 'Terrain' and is usually available on the island but to be on the safe side you may want to buy it online before you go. Either direct from the publisher:
http://www.terrainmaps.gr/ or from places like Amazon.
Foxy Island Walks
http://www.foxysislandwalks.com/Symi-Greece.html has a lot of information about Symi's beaches and some very good and easy to follow walking guides.
There used to be a book by Lance Chilton about walks on Symi but it's been out of print for some time.
http://www.amazon.co.uk/Lance-Chilton/e/B004M3NM7E/ref=dp_byline_cont_book_1 You might be able to track one down online somewhere. Alternatively, there is a similar book written by a Symi resident and that should be available on the island. It's by Frances Noble and you should be able to get it from Kalodoukas Holidays office.