RW was an active correspondent, often writing several letters a day. It
has been estimated that he wrote over 10,000 letters during his lifetime. 
Unfortunately, Cosima Wagner destroyed many unpublished letters, including 
the originals of Richard's letters to Mathilde Wesendonck, those from Otto 
and Mathilde to Richard, Nietzsche's letters to Cosima, Peter Cornelius' 
letters to Richard, those written to him from Pusinelli, Röckel, H. Heine, 
Berlioz, Herwegh, Semper, Gasperini, Baudelaire, Nietzsche, Gobineau and 
H. von Stein, and all of the correspondence with Hans von Bülow in the 
period preceding and immediately after their divorce. Later Cosima even 
burned many of Richard's letters to herself. Wagner himself had destroyed 
letters from Judith Gautier.  Further bundles of correspondence were 
incinerated by Eva Wagner in 1909.

Many of Wagner's letters have been published, usually in a separate volume
for each correspondent; for example, the letters between RW and Mathilde
Wesendonck (an important resource for students of 'Tristan', 'Die Sieger'
and 'Parsifal') were published in Berlin, 1904, with an English
translation (by Ellis) of them published in London, in 1905.  Also
important is the collection of correspondence between RW and his patron,
King Ludwig II of Bavaria, 5 vols., edited by Otto Stroebel and published
1936-39, Karlsruhe.

In 1967, work began on a complete edition of the existing letters in their
original languages.  At that time, the editors anticipated an edition of
fifteen volumes, but in the introduction to volume 6, they revised their
estimate to 30 volumes, containing between 7000 and 7500 letters.  The 
project is now being led by Dr. Werner Breig.  More than 500 letters in 
English translation have been edited by Stewart Spencer and Barry 
Millington.