There are many resources for additional information on copyright and permissions:
Public Domain Chart
Essential tool for calculating when U.S. works pass into the public domain.
Association of American University Presses (AAUP) Copyright & Permissions Resources
Excellent source for information on copyright and permissions, especially as they relate to scholarly publishing.
Copyright Clearance Center (CCC)
Permission agency online site. Their copyright resources section includes compliance guidelines, articles on rights management, and links to many useful sites including other intellectual property associations and licensing organizations.
WATCH File (Writers, Artists, and Their Copyright Holders)
Database of copyright holders whose archives are located in libraries and archives in North America and the United Kingdom.
Resource for locating hard-to-find authors for permissions requests. Searchable by email inquiry.