In reply to Stephen Reid - Needle Sports:
It's also very out of date as there are many missing or the information is incomplete/incorrect (e.g. where to rent gear etc), so I think the online sources are the best.
This has been fairly up to date and recently re-vamped;
http://www.viaferrata.org
Also, for Switzerland, the Swiss travel website is actually quite accurate and up to date for information on location etc.
http://www.myswitzerland.com/en-ch/via-ferrata-in-switzerland.html
If in switzerland, just be careful if you try to rent anything over lunch time... everything is closed 12-2 generally...!