The Theatetus is Plato's most extended discussions of the theory of knowledge. The absence of the theory of forms from the dialogue has perplexed commentators, some of whom conclude that Plato abandoned the theory in the light of the criticisms adduced in the Parmenides.
Free Online and Downloadable Texts
Gutenberg: Theatetus, translated by Benjamin Jowett, multiple formats.
University of Adelaide (Internet Archive): The Theaetetus, translated by Benjamin Jowett. EPUB, HTML and MOBI formats.
Wikisource: Theaetetus, translated by Benjamin Jowett, 1871. Online text.
History of Philosophy without any gaps: I Know, Because the Caged Bird Sings: Plato's Theaetetus - podcast by philosopher Peter Adamson.
Librivox: Theaetetus - audiobook.
Internet Encylopedia of Philosophy: Plato - Theaetetus.
PhilPapers: Plato - Theaetetus - bibliography with open access option.
Stanford Encyclopedia of Philosophy: Plato on Knowledge in the Theatetus.
The Great Conversation: Further reading at Tom's Learning Notes
Ancient Greek resources: Learn to read Greek classics in the original.
Bloom's Western Canon: Plato's Dialogues are listed.