http://webhome.idirect.com/~totton/abacus/PDF.htm
Seems like the best free resource.
This book seems like the standard english text on soroban -
https://smile.amazon.com/Japanese-Abacus-Its-Use-Theory/dp/0...
The first few chapters are great. This book from the sixties mentions that abacus are actually a middle-eastern invention and came east with trade.
Abacus in fixed frames are a later invention for speed but earlier versions would have more resembled a backgammon table using pebbles.
https://en.wikipedia.org/wiki/Abacus
https://en.wikipedia.org/wiki/Salamis_Tablet
This led to this pretty interesting paper -
https://arxiv.org/pdf/1206.4349.pdf
It seems like there is a pretty large gap in our understanding of how people actually did calculation?
Cheers