Particularly useful for those beginning their study in a field, OBO brings together key literature on a topic from top scholars in the field with the goal of giving a "guided tour" through the literature that includes context and a balanced overview.
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Contrary to its name, this work is actually a 12 volume encyclopedia covering the Western and Eastern Europe, Asia Minor, and Muslim-Christian North Africa from 500-1500 C.E. Although somewhat dated, it can still be useful.
Medieval Iberia: an encyclopedia by E. Michael Gerli, editor