Prof Speer's website with a (long!) list of his publications. For his work on medieval aesthetics see for instance:

• A. Speer, "Art as Liturgy. Abbot Suger of Saint-Denis and the Question of Medieval Aesthetics", in: J. Hamesse (ed.), Roma, magistra mundi. Intineraria culturae mediaevalis (Louvain-la-Neuve: 1998), 855-75.

• A. Speer, "Beyond Art and Beauty. In Search of the Object of Philosophical Aesthetics", International Journal of Philosophical Studies 8 (2000), 73-88.

• A. Speer, "Aesthetics", in: J. Marenbon (ed.), The Oxford Handbook of Medieval Philosophy (Oxford: 2012), 661-84.