The publication of diaries by Cosima Wagner, that had long been suppressed
by the Wagner family, has greatly increased our knowledge of Richard and
Cosima Wagner and their life together. Also Richard's own diaries/
notebooks are of interest. The Wagner diaries are the following: 

* 'Die Rote Brieftasche' in 'Sämtliche Briefe', ed. G. Strobel and W. Wolf,
  1967. Wagner's Red Pocketbook, containing his autobiographical notes for 
  the years 1835 to 1839. Notes from later years were included in the 
  'Brown Book' as the 'Annals'. 

* 'Das Braune Buch: Tagebuchaufzeichnungen, 1865 bis 1882', ed. Joachim
  Bergfeld, 1975. Wagner's diary and notebook, which he used at various 
  times between 1865 and 1882. English translation by George Bird, 1980, 
  as 'The Brown Book'. 

* 'Cosima Wagner: Die Tagebücher' 1869-1883, 2 vols. hardback, 4 vols.
  paperback. Edited by Martin Gregor-Dellin and Dietrich Mack, 1976-77. 
  English translation in 2 volumes by Geoffrey Skelton, 1978-1980; out of 
  print, but a condensed version is available in one volume. 

Details of English editions of the above are given in the Wagner Books FAQ.