Zen is a school of Buddhism in the Mahayana tradition, but the Mahayana still relies upon the teachings and the lessons of Theravada Buddhism. The Tipitaka (Pali ti, "three," + pitaka, "baskets"), or Pali canon, is the collection of primary Pali language texts which form the doctrinal foundation of Theravada Buddhism. The Tipitaka and the paracanonical Pali texts (commentaries, chronicles, etc.) together constitute the complete body of classical Theravada texts.
The Pali canon is a vast body of literature: in English translation the texts add up to thousands of printed pages. Most (but not all) of the Canon has already been published in English over the years. Although only a small fraction of these texts are available on this website, the collection can be a good place to start.