Dupuis,
     the publisher of Yoko (French and Dutch)

     Cinebook,
     the publisher of Yoko in English

http://www.yokotsuno.com
Yoko's official site (in French)

https://yokotsuno.fr/ (ex http://yoko.tsuno.free.fr/)
A wondeful site about Yoko, check its forum (in French)

http://passionnementyoko.free.fr
A site of a passionate (in French)

http://www.yokotsuno.fi/
Finnish site with plenty of details and crossreferences

http://en.wikipedia.org/wiki/Yoko_Tsuno
Wikipedia's page for Yoko

http://www.dickgrune.com/Cult/Yoko_Tsuno/
Notes on the Dutch translation of Yoko's books as well as The Yoko Tsuno File (information about her name, familie, place of birth,...) by Dick Grune

https://forum.nolife-tv.com/threads/bande-dessin%C3%A9e-yoko-tsuno.8419/ (from Internet archive)
Yoko Tsuno's thread on the ex TV channel Nolife's forum (French)

http://code_phare.voila.net/yoko.htm (from archive.is)
Page about "On the Edge of Life" (in French)

http://www.okaze.de/yoko/ (from Internet archive)
German site

https://www.parnass.scram.de/
Books reviews in German, including The Celestial Junk, The Pagoda of the Mists, The Seventh Code, Integral 1&2, The Amethyst

http://ani.donmai.ch/?p=326
Swiss site in German, mention a project of animé on Yoko

http://jim-on-the-web.com/comic_errors.php?comic=YokoTsuno (from Internet Archive)
Page that lists errors in Yoko's books (in English)

http://www.cachislamar.com/yokotsuno/home.htm
Spanish site about Yoko

http://www.writeups.org/fiche.php?id=5641 (Yoko),
http://www.writeups.org/fiche.php?id=5642 (Monya)
Excellent articles by Sébastien Andrivet (in English)

https://sites.google.com/site/tenuesdeyoko/Home (from Internet Archive)
Site that lists Yoko's clothing (in French)

https://www.actusf.com/detail-d-une-edition/2012-10-16-jeunesse-serie-yoko-tsuno
Article on Yoko on actusf (in French)

http://www.cinebook.co.uk/index.php?cPath=189
Books list at Cinebook

https://storyhouseegmont.fi/
Story House Egmont, the publisher in Finnish of Yoko
(blog)

https://www.carlsen.de/autorin-illustratorin/roger-leloup
Books list at Calsen, the publisher in German of Yoko

http://www.cobolt.dk/
Cobolt, the publisher in Danish of Yoko

http://www.renoircomics.it/
ReNoir Comics, in Italian

http://ponentmon.com/
Ponent Mon, in Spanish