2nd-person plural used when addressing one person
 

In reading historical novels, I often see the 2nd-person plural (informal) used when the person speaking is obviously addressing just one person. Was this common in "historical" times? The writing is contemporary Spanish but the usage seems archaic.

You can check here and here some previous discussions on this topic. :)

¡Gracias! No he podido encontrar algo en la historia de la lengua castellano sobre el uso de sois, habéis, decís etc. como singular formal.

I don't know if there is any material in English about it, but in Spanish you can search book titles like "historia de la gramática española" or "historia del español".
And there is a little historical note on the use of "vos" and "vosotros", in the Real Academia Española's grammar manual, the "Nueva Gramática de la Lengua Española".