Chapters one, two, and four of De ente et essentia by Thomas Aquinas. Link to text here.
Section six, "Essence and esse," pages 99-107 in "Metaphysics" by John Wippel, in The Cambridge Companion to Aquinas. Link to articlehere and here.
Further Reading
Optional reading:
The remaining chapters of De ente et essentia.
Chapter five, "Essence and existence in the De ente et essentia, ch. 4," and chapter six, "Essence and existence in other writings," pages 107-61 in Metaphysical Themes in Thomas Aquinas, vol. 1, by John Wippel. Link to text here and here.