Links to some of the numerous online resources for the 1990 World Cup:
fifa.com: The official web site for the tournament
wikipedia.org: A complete overview, incl. some narratives
arquivosdosmundiais.br: Match information (incl. qualifiers)
allworldcup.narod.ru: Match information (incl. qualifiers)

Books and podcasts on Italia’90 (all highly recommended):
Simon Hart, World in Motion: The Inside Story of Italia’90
Pete Davies, All Played Out: The Full Story of Italia’90
Vincerà: The Story of Italia’90

Films
Anwar Rome: Documentary about UAE’s qualifiers for Italia’90

Articles on the (contested) legacy of the 1990 World Cup:
Amy Lawrence, The Guardian (15 June 2015)
Steven Scragg, thesefootballtimes.co (27 July 2015)

A useful source on various nostalgia:
soccernostalgia.blogspot.com

Videos/DVDs:
‘Paul Breitner’ blogspot (thousands of available items) 
Soccervideos EU

(We’re indebted to both for providing us with some hidden gems)