This PhD thesis is a careful discussion of Aristotle's Metaphysics VII (Zeta) almost line by line (or at least argument by argument). It might be interesting to know that a revised and shortened version of it was published as a book in Portuguese, "As Noções Aristotélicas de Substância e Essência", Campinas: Editora da Unicamp, 2008. Actually, I believe the original version of the PhD thesis has been superseded by the book.