An article about Old Maps Online, by Humphrey Southall and Petr Pridal, has just been published in the open access journal e-Perimetron. You can read it from here:
https://www.e-perimetron.org/Vol_7_2/Vol7_2.htm
It is based on the presentation we gave at the International Cartographic Association meeting in Barcelona back in April. The article is not so much about the technology behind the portal as about our wider aims, and how we are working with map libraries to achieve them.