The CONTU guidelines were established in 1978 to add clarifications to the original copyright law governing ILL. The "Rule of 5" originates from CONTU, and is suggested as best practice in order to avoid copyright infringement. This links to the complete text of the CONTU guidelines.
When providing article copies through ILL, check that there is a copyright statement visible. If there is no notice, you can attach the journal issue's copyright notice or use this standard notice of copyright.
The University of Texas Libraries maintains a Copyright Crash Course with a section specifically for librarians. It includes information on Section 108 as well as fair use and licenses for digital resources.
A wiki managed by ALA with many resources including links to copyright law and relevant copyright resources. The wiki also has information on fair use, the First Sale Doctrine, videos / movies in the library, and the TEACH act.