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.
A nice overview of participants in European Cups until 2002
is made by zwartepiet aka santiclaus. Because the original website
was inactive for some time, a
mirror has been made. Another overview of the history of soccer is made by pluto.