1. General: "Roofing Solutions: Reference Manual for Property Managers, Owners, Architects, and Specifiers" is an excellent introductory guide to roof maintenance, and effectively explains the bottom line value of a roof maintenance program. It is made available by the GAF Materials Corporation.
2. General: A decent general roof inspection and maintenance guide for low-slope roofs is available here at the Firestone Building Products website.
3. Slate Roofs: "The Repair, Replacement and Maintenance of Historic Slate Roofs" is a comprehensive resource available on the US National Park Service's Technical Preservation Services website.
4. Tile Roofs: "The Preservation and Repair of Historic Clay Tile Roofs" is another comprehensive resource available on the US National Park Service's Technical Preservation Services website.
5. Wood Shingle and Shake Roofs: "Care and Maintenance of Wood Shingle and Shake Roofs" is worth taking a look at. Provided on Oregon State University's Pacific Northwest Plant Disease Management Handbook site.