Historical and current information, commentary, books, articles, and Jos Couman’s invaluable Concordance of the Rubáiyát are contained in these sites:

omarkhayyamnederland.com

omariana.nl

https://rubaiyatconcordance.org

https://omarkhayyamrubaiyat.wordpress.com

omarkhayyamrubaiyat.com

http://www.bobforrestweb.co.uk/The_Rubaiyat/index_of_the_rubaiyat_archive.htm

https://theomarkhayyamclubofamerica.wordpress.com