Subject searches to try in SOLO for general works
Libri feudorum
Feudal law - Europe
Feudalism - Europe
Power (Social sciences) -- Europe -- History -- To 1500
Estates (Social orders) -- Europe -- History -- To 1500
A sample of old & new with English (or Englished) works first: a lot will be in a Bodleian History Reading Room rather than the Law Bod.
Fiefs and Vassals : the medieval evidence reinterpreted by 
Feudal Society by 
Feudalism by 
Die Gegenwart des Feudalismus by The name of a Milanese tenure judge, Obertus de Orto (fl 1150), has become associated with a summary of the feudal system of Langobardia/Lombardy, the Libri Feudorum. As these were appended to Justinian's CJC, Italian feudal law was known to the faculties of universities as these institutions spread across Europe.
Evidence for the operation of feudal laws in other regions also survive. See MA:country tabs in this guide
The Libri Feudorum (the ‘Books of Fiefs’) : An Annotated English Translation of the Vulgata recension with Latin Text by 