There are very few well fundamented sources of information. Original literature is not always proof. Fake information, which was launched to mislead competidors, is stil around and the origin of myths and contradictions like the number of produced instruments, and whuich is far from the truth.

Similarly many published articles refer to the same uncertain sources. The phd tesis of Maria Dunkel is a trustable source.

Bandonion und Konzertina, Maria Dunkel (in German)
Berliner Musikwissenschaftliche Arbeiten,
Musikverlag Emil Katzbichler, München / Salzburg, 1987 (now newer edition)
ISBN 3-87397-070-8

El Bandoneón en el Tango, Oscar D. Zucchi (in Spanish)
La Historia del Tango, tomo V, El Bandoneón,
Ediciones Corregidor, Buenos Aires, 1977
You may try to get it at on-line book store.

Bandoneon in Uruguay, Maria Dunkel (in German)
private edition, Lenzburg / Berlin, 1994

Bandoneon Pure – Dances of Uruguay SF 40431
80 page CD-booklet with a translated extract of
Maria Dunkel Bandoneon in Uruguay
Smithsonian Folkways - Traditional Music of the World 5
Smithsonian / Folkways Recordings
Office of Folklife Programs
955 L'Enfant Plaza, Suite 2600
Smithsonian Institution
Washington, DC 20560

Akkordeon, Bandonion, Concertina im Kontext der Harmonikainstrumente
Maria Dunkel (German)
, 1999,
Ralf Kaupenjohann, Bleckstrasse 1a, D-44809 Bochum
ISBN 3-924272-05-0

Der Musikwinkel und die Harmonika
Kurt Kauert (German)
Reihe Weiss - Grün 22
Sächsische Landesstelle für Volkskultur
Schneeberg / Erzgebirge
Druck-und Verlagsgesellschaft Marienberg mbH
ISBN 3-931770-28-1

Meantone is beautiful! - Studies on Tuning of Musical Instruments. Reinhard Frosch,
Bern, Berlin, Bruxelles, Frankfurt/M., New York, Oxford, Wien, 2002. 254 pp., num. fig. and tables;
154 exercises with solutions.
ISBN 3-906769-92-5 / US-ISBN 0-8204-5927-5 pb.