Logic

Free to read here, from Project Gutenberg.

A System of Logic, Ratiocinative and Inductive

John Stuart Mill, also an audiobook

The Advancement of Learning

Francis Bacon

A System of Logic, Ratiocinative and Inductive (Vol. 1 of 2)

John Stuart Mill

The Logic of Hegel

Georg Wilhelm Friedrich Hegel

The Essays of Arthur Schopenhauer; the Art of Controversy

Arthur Schopenhauer

The Categories

Aristotle

The principles of science

William Stanley Jevons

Wissenschaft der Logik — Band 1

Georg Wilhelm Friedrich Hegel

German

The Game of Logic

Lewis Carroll

Logic: Deductive and Inductive

Carveth Read

A System of Logic: Ratiocinative and Inductive, 7th Edition, Vol. I

John Stuart Mill

Euthydemus

Plato

Essays in Experimental Logic

John Dewey, also an audiobook

The Art of Logical Thinking; Or, The Laws of Reasoning

William Walker Atkinson

A Class Room Logic

George Hastings McNair

Studies and Exercises in Formal Logic

John Neville Keynes

A System of Logic: Ratiocinative and Inductive, 7th Edition, Vol. II

John Stuart Mill

Einführung in die moderne Logik. Erster Teil.

Goswin Uphues

German

The Mathematical Analysis of Logic

George Boole

Logic, Inductive and Deductive

William Minto

A Logic of Facts; Or, Every-day Reasoning

George Jacob Holyoake

The positive outcome of philosophy

Joseph Dietzgen

Prolegomena to the Study of Hegel's Philosophy, and Especially of His Logic

William Wallace

Studies in Logical Theory

John Dewey

Wissenschaft der Logik — Band 2

Georg Wilhelm Friedrich Hegel

German

Logic as the Science of the Pure Concept

Benedetto Croce

Deductive Logic

St. George William Joseph Stock

John Dewey's logical theory

Delton Thomas Howard

Reformed Logic

D. B. McLachlan

First notions of logic (preparatory to the study of geometry)

Augustus De Morgan

Logica

Andres Piquer

Spanish

Ευθύδημος

Plato

Greek

Grundriß der Logik

Kurt Joachim Grau

German

The Essentials of Logic, Being Ten Lectures on Judgment and Inference

Bernard Bosanquet

Lehre vom Schluss

Aristotle

German

Topik

Aristotle

German

Picture logic

Alfred James Swinburne