Please see the very useful review of many of the textbooks below by Curtis Franks.

• Craig DeLancey, A Concise Introduction to Logic (PDF, HTML, EBUB; CC BY-SA 4.0). With online proof checker based on Kevin Klement's

• Antony Eagle, Elements of Deductive Logic, GitHub source code (PDF; CC BY-SA 4.0)

• Graham Leach-Krouse and Jake Ehrlich, The Carnap Book, online with computer support for exercises (HTML; CC BY 4.0)

• P. D. Magnus, forall x (PDF; CC BY 4.0, open source) GitHub source code. This version is supported by Carnap.

  Translations of forall x:

  • Hungarian: forall x: Pecs (CC BY 4.0)
  • Spanish: paratodo x: Una Introducción a la Lógica Formal (CC BY-SA 3.0)

  Derivatives:

  • Tim Button, forall x: Cambridge (CC BY 4.0) GitHub source code

    Translations of forall x: Cambridge:

    • Icelandic: fyrir öll x (CC BY 4.0)

    Derivatives based on forall x: Cambridge:

    • Antony Eagle, forall x: Adelaide (CC BY 4.0) Github code

    • Kathryn Lindemann, forall x: SLU (CC BY-SA 3.0)

    • Michael Rieppel, forall x: SYR (CC BY-SA 4.0) GitHub source code

    • Aaron Thomas-Bolduc, Richard Zach, forall x: Calgary (CC BY 4.0; also includes chapter on truth-functional completeness and soundness from Button's 'Metalogic' and chapters on semantics and proof theory of modal logic from Rob Trueman), GitHub source code. This version is also supported by Carnap and also available in HTML with accessibility features.

      Translations of forall x: Calgary:

      • Portuguese: Daniel Durante, Maria da Paz Nunes de Medeiros, Para Todxs: Natal (CC BY 4.0)
      • German: Simon Wimmer, forall x: Dortmund (CC BY 4.0; translation into German without chapters on metatheory)

      Derivatives based on forall x: Calgary:

      • Catrin Campbell-Moore, forall x: Bristol (CC BY-SA 4.0)
      • Colin Caret, forall x: Utrecht (no download or source)
      • J. Dmitri Gallow, forall x: The Pitt Edition (CC BY 4.0)
      • Patrick Girard, Andrew Withy, Jeremy Seligman, forall x: Auckland (CC By 4.0). Significantly revised, combines elements of the Cambridge and Calgary versions and uses the truth trees of the UBC version for proofs. No modal logic or metatheory.
      • Greg Johnson, forall x: Mississippi State edition (CC BY 4.0)
      • Davis Smith, forall x: R^3 (CC BY 4.0). Significantly revised for a 10-week quarter course.
      • Marcus Rossberg, forall x: UConn (CC BY 4.0; includes a chapter on non-classical logic but leaves out the chapters on metatheory and modal logic)

    • Proof checker for forall x: Cambridge and derivatives by Kevin Klement

  Derivatives of forall x, continued:

  • Nathan Carter, forall x in Lurch (CC BY-SA 3.0; for use with the Lurch mathematics editor, which can check proofs!), [No source code]
  • Edward Elliot, forall x: Leeds (CC BY-SA 3.0; also includes chapters on probability theory), Source code Zip
  • Jonathan Jenkins Ichikawa, forall x: UBC, GitHub source code (CC BY 4.0), includes trees in addition to natural deduction proofs
  • J. Robert Loftis and Cathal Woods, forall x: Lorain County Remix, GitHub source code (CC BY-SA 3.0; also includes chapters on informal logic)

• Matthias Felleisen, Phokion Kolaitis, Ian Barland, John Greiner, and Moshe Vardi, Intro to Logic (CC BY 1.0)

• Vann McGee, Logic: The Art of Persuasion and the Science of Truth (CC BY-NC-SA 4.0)

• Derek Turner, Form and Content: An Introduction to Formal Logic (CC BY-NC-ND 4.0)

• Matthew J. Van Cleve, Introduction to Logic and Critical thinking (CC BY)

• Sean Walsh, Introductory Logic (CC BY-SA-NC 4.0; an open text fully based on Carnap)

• Nuel Belnap, Notes on the Art of Logic
• Johan van Benthem, Hans van Ditmarsch, Jan van Eijck, Jan Jaspars, Logic in Action
• Josh Dever, Get Rational With These Four Weird Tricks!
• Gary Hardegree, Symbolic Logic: A First Course
• Gary Hardegree, Symbolic Logic: A Second Course
• Terrence Parsons, An Exposition of Symbolic Logic (text goes with David Kaplan's Logic 2010 software and grading system
• Tony Roy, Symbolic Logic: An Accessible Introduction to Serious Mathematical Logic, vol. I
• John Slaney, The Logic Notes
• John Slaney, Logic for Fun
• J. David Velleman, blogic (online, interactive)
• Stefan Waner and Steven R. Costenoble, Introduction to Logic

• Howard Pospesel and David Marans, Arguments: Deductive Logic Exercises (2nd ed, 1978, Prentice Hall) (alternate download site)
• Paul Teller, A Modern Formal Logic Primer (1989, Prentice Hall)

• Open Logic Project, The Open Logic Text (CC BY 4.0, open source), with derivatives:
  • Tim Button, Set Theory: An Open Introduction (CC BY 4.0, open source)
  • Michael Hallett, Intermediate Logic (CC BY 4.0, open source)
  • Audrey Yap and Richard Zach, What If? An Open Introduction to Non-Classical Logic (CC BY 4.0, open source)
  • Richard Zach, Sets, Logic, Computation: An Open Introduction to Metalogic (CC BY 4.0, open source)
  • Richard Zach, Incompleteness and Computability: An Open Introduction to Gödel's Theorems (CC BY 4.0, open source)
  • Richard Zach, Boxes and Diamonds: An Open Introduction to Modal Logic (CC BY 4.0, open source)
• Tim Button, Metatheory (CC BY-SA 3.0, open source)
• Christopher C. Leary and Lars Kristiansen, A Friendly Introduction to Mathematical Logic (CC BY-SA-NC 3.0, PDF only)
• Dave Witte Morris and Joy Morris, Proofs and Concepts (CC BY-NC-SA 2.0, open source)

• Nuel Belnap, Notes on the Science of Logic
• Gary Hardegree, Introduction to Metalogic
• Gary Hardegree, Modal Logic
• Gary Hardegree, Compositional Semantics
• Gary Hardegree, Set Theory
• Jeff Paris, A Short Course in Predicate Logic
• Tony Roy, Symbolic Logic: An Accessible Introduction to Serious Mathematical Logic, vol. II
• Stephen G. Simpson, Lecture notes on foundations, model theory, incompleteness, undecidability.

• Kai von Fintel, Irene Heim, Intensional Semantics (CC-BY-NC-SA 3.0)
• Steve Yablo, Modal Logic (CC-BY-NC-SA 3.0)
• Richard Zach, Boxes and Diamonds: An Open Introduction to Modal Logic (CC BY 4.0, open source)

• Richard Pettigrew and Jonathan Weisberg, The Open Handbook of Formal Epistemology (CC BY-NC-ND 4.0)

• Wolfgang Schwarz, Belief, Desire, and Rational Choice (CC BY-NC-SA 4.0)

• Michael Strevens, Notes on Bayesian Confirmation Theory

• Brian Weatherson, Lecture Notes on Decision Theory (CC BY-NC-SA 4.0, open source)

• Jonathan Weisberg, Odds & Ends: Introducing Probability & Decision with a Visual Emphasis (open source)

"Free" means: can be downloaded or accessed online for free. "Open" means: available under an open license, i.e., modification and distribution is permitted. "Open source" means the source files from which PDFs can be produced are available.

See also the tracking issue #41 for possible inclusion of materials already freely available.