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Various legal treatises on Roman/Civil Law, Tapia Ramírez’ ‘Bienes y Derechos Reales’ is particularly good but not sure it’s available in English.
Edit- if you really want to know I can explain the etymological origins (as I remember them) tomorrow. Basically it originated to refer to the physical (aka real, as is covered by droit réel, not as in‘not imaginary) part of a person’s patrimony (aka estate).
@gbj_tester, @Hjusth: do you have any evidence to support your claims?