This list of works is by no means exhaustive. It contains just those of which I know, and those which have been pointed out to me. Books in French are here.

Thank you to Gail for her help

Beethoven as I knew him

One of the first biographies
Author: Antoine SCHINDLER
Dover Publications
Annotated by Donald W. MacArdle
1996 - 548 pages
ISBN: 0-486-29232-0.

Beethoven remembered, biographical notes

Written by friends of Beethoven
Authors: Ferdinand RIES et F.-G. WEGELER
Great Ocean Publishers
1987 - 200 pages
ISBN: 0-915556-15-4.

Memories of Beethoven

The memoires of the young son of a friend of Beethoven, who was often with Beethoven at the end of his life
Author: Gerhard von BREUNING
Translation in English by Henry Mins and Maynard Solomon
Cambridge University Press
1995 - 154 pages
ISBN: 0-521-48489-8.

Beethoven: Impressions by his Contemporaries

Letters, diaries, memoirs, etc., by Rossini, Weber, Liszt and friends...
Author : Oscar SONNECK
Dover Publications
238 pages
ISBN: 0486217701.

The Life of Beethoven, including his correspondence with his friends

Aquaintences and Letters
Author: Iganz MOSCHELES
Publication: London: H. Colburn, 1841
2 volumes.