In addition to the books mentioned in previous titles in this series, I found the following very useful:
Ellrich, Hartmut. Die Salier: Die Geschichte der Herrscherdynastie. (Petersburg, Germany: Michael Imhof Verlag GmbH, 2011).
Zuch, Rainer. Pfalzen Deutscher Kaiser: Von Aachen bis Zürich. (Petersburg, Germany: Michael Imhof Verlag GmbH, 2007).
The Holy Roman Empire did not have a capital city per se until the 1400s, when Vienna became the defacto headquarters of the emperors. As a result, the Carolingian, Salian, and Staufen rulers moved from place to place, along with their courts and armies. Rather than straining the resources of various towns, monasteries, and nobles, establishments called pfalzen (or a pfalz) served as rest-stops, supply depots, and government offices.
Also by Alma T. C. Boykin
Merchant and Magic - Merchant and Empire Book One
The Scavenger’s Gift - Merchant and Empire Book Two (short story)
Imperial Magic - Merchant and Empire Book Three
Merchant and Empire Page 19