One indispensible reference work is:

• Griffiths, Paul: The String Quartet, Thames and Hudson, 1983

Some references on individual composer's quartets:

• Kerman, Joseph: The Beethoven Quartets
  Oxford University Press, London, 1967
  
• Barrett-Ayres, Reginald: Joseph Haydn and the String Quartet
  Barrie & Jenkins, London, 1974
  ISBN 0-214-66803-7
  
• Rauchhaupt, Ursula; ed: Schoenberg, Berg, Webern: The String Quartets
  Deutche Grammophon Gesellschaft, Hamburg, 1971
  

Books on quartet performance and performers:

• Blum, David: The Art of Quartet Playing: The Guarneri Quartet in Conversation
  Victor Gollancz, London, 1986
  ISBN 0-575-03930-2
  
• Snowman, Daniel: The Amadeus Quartet: The Men and the Music
  Robson Books, London 1981
  ISBN 0-86051-106-5
  

Some general references with special relevance for quartet literature:

• Cobbett, Walter Willson; ed; Colin Mason; ed: Cobbettt's Cyclopedic Survey of Chamber Music (2nd ed, 3 vol)
  Oxford U Press, London, 1963
  

Version 2.4: August 17, 2000