I'm reading
The European Nobility in the Eighteenth Century which is a series of essays on the major European noblities as they existed in the 1700's. Hopefully I can turn something up interesting. The distinctions between those who have privilage and those who have power is interesting. I think GURPS needs a book on the fine points of status, something between Social Engineering and Power-ups.
Still, minor tinkering with the meaning of noble status could have major effects on a society. This would make an interesting set of variations.