Preserving Books and Papers:
The Northeast Document Conservation Center's Preservation of Library and Archives Materials: A Manual provides a well-written series of short articles on various preservation topics.
The Abbey Newsletter, edited by Ellen McCrady, is one of the "must reads" in book and paper conservation.
Conservation OnLine (CoOL) is targeted to professionals, so much of the information is technical, but a great deal of information is available.
Preserving and Handling Photographic Images:
Care of Photographs, also from NEDCC, provides excellent advice on the handling and storage of photographic materials.
Preserving Electronic and Magnetic Media: