Norwich is fun, but he very much belongs to an older generation of history writing, which people read fore literary enjoyment as muichas education. Not that he'swrong, hejust focvuses way too much on the grand narrative.
I like Donald Matthew: The Norman Kingdom of Sicily for a nuts-and-bolts-vierw of how the place was governed. It's a little older already, but the bibliography is good until the 90s. David Abulafia's biography of Frederick II was also quite good, but it only touches on the late Norman state. I don't know if the exhibition catalogues from Vienna (nobiles officinae) and Oldenburg (/Friedrich II) ever were translated. They are good, too. I'm prettysure the current staufer exhibit in Mannheim will have even newer material, but Ihaven'tz gotten round to reading the catalogue yet. it focuses very much on the comparative economic and social history of Sicily, Northern Italy and West Central Germany in the 12th and 13th century.