Specifics on Diacritics:
See also Characters.
Character design standards @ Microsoft
Data on Languages @ Institut of Esthonian Language
Diacritics Project. All you need to design a font with correct accents
Gaultney, J. Victor: Problems of diacritic design for Latin script text faces, 2002
Polish Diacritics How To?
Thorn and eth: how to get them right
Wells, J.C.: Orthographic diacritics and multilingual computing
Context of Diacritics by Ondrej Jób
On Diacritics by David Březina