The rec.sport.soccer FAQ: Qualification and Seeding for the European Cups of Martin Protzen. The FAQ was the most complete background information on the qualification and seeding for the European Cups for many years, but is not updated anymore.
An overview of all clubs in the database at this website may be found at clubs participating in European Cups since 1955.
The match results of all European Competitions since 1955 can be found in the European Cups Archive of James M. Ross. Match results including the dates of the matches may be found at statto.com.
The history of UEFA rankings from 1978 till 1998 is prepared by Pawel Mogielnicki. See here for a local copy.
The European Cup history website has lots of details on the history of the European Cups since 1955. A dated overview of participants in European Cups until 2002 is made by santiclaus. Because the original website is not available anymore, a mirror has been made. Another dated overview of the history of soccer is made by pluto.