Categories and Commentary, crucial background thereto. Call me a flat-footed old philologist, but Aristotle’s as vital and relevant to argument, discussion, glossing, and criticism today as the works were to the 13th c. or indeed the 4th BCE … Not the world’s best translations, but a fair starting point, and at least they’re free and freely available (and there’s some Greek, and some later commentaries).

The Internet Classics Archive
Perseus Digital Library
Project Gutenberg