Stanford Encyclopedia of Philosophy: Leibniz
http://plato.stanford.edu/entries/leibniz/
This is a summary of Leibniz's philosophy. Of particular note is the section on the Monadology, which is repeatedly referenced in Borges's stories.
Stanford Encyclopedia of Philosophy: Spinoza
http://plato.stanford.edu/entries/spinoza/
Spinoza is another philosopher who Borges references often in Ficciones. He created a strange idea whereby the divine and the human exist in parallel yet forever separate realms, with some exceptions.
Borges: Interview on the Art of Writing
http://www.theparisreview.org/interviews/4331/the-art-of-fiction-no-39-jorge-luis-borges
This interview asks...
