A. Wolf 
Essentials of Logic [PDF ebook] 

Originally published in 1926, this book is an exploration of the essentials of logic: the study of the general conditions of valid inference. The main aim of logic is not to teach people to reason correctly, but to explain what happens when they do reason correctly, and why some reasoning is not correct, and this book contains chapters examining judgment and terms; categorical propositions and their implications; and deduction and syllogism.