One more book you might be interested in
Keeping it Clean-Removing Exterior Dirt, Paint, Stains and Graffiti from Historic Masonry Buildings
Anne E Grimmer
U.S. Department of the Interior National Park Service
Preservation Assistance Division Technical Preservation Services
Available here:
keeping it clean - Google Book Search
Its an excellent technical resource for any pressure washer cleaning masonry surfaces. It explains acid and alkaline cleaners, poulticing methods, removing bird droppings. Chemistry lovers should really enjoy it