Interesting books on Wagner Singers:
The only book I know of on the tenor Wolfgang Windgassen is in German.
The Ring Resounding - John Culshaw (discusses the making of the Solti Ring)
A Hero's Life - Jeannie Williams (biography of Jon Vickers)
George London - of Gods and Demons by Nora London (biography of George London)
Divo - Helena Mathenopolous - this is about various male singers and their career
Diva - (see above, but female singers)
55 Years in Five Acts - Donald Arthur (biography of Astrid Varnay)
Tristanissimo - The Authorized Biography of Lauritz Melchior there is also a book by Melchior's son Ib Melchior
Yankee Diva - Ira Glackens (Biography of Lillian Nordica)
My Life in Opera - Birgit Nilsson
Foreign Parts - Thomas Allen (memoirs of this great baritone who has sung the role of Sixtus Beckmesser)
That's all I've got in my memory, others may have their favorites.
If you're looking for a "fun" read - try Tom Holt's "Expecting Someone Taller". It's a humorous book which answers all the "burning" questions regarding the Ring characters who lived on after Götterdämmerung.
A great source for used books is
www.abe.com This is a vast organization of booksellers who post their used, out-of-print and rare books on-line. I have been ordering through them for many years and have had very good experience with the booksellers both foreign and domestic.
Another source is addall.com and
Used@addall.com I believe
Used@addall.com also searches through
www.abe.comBest,
Flosshilde