"Introduction to Logic", Irving M. Copi. - It was the very first logic book I read. It's kinda old, and treat some subjects like basic logic, language, definitions, fallacies, arguments, validity, syllogism, symbolic logic, natural deduction, inference rules, propositional functions, quantifiers and induction. It has a lot of exercicies, with the answer in the end of the book.
"Introduction to Logic", Harry Gensler - Newest, this book has a more didatic approach, and contains everything that Copi's has, and relations; identity, modal, deontic, imperative and belief logic; metalogic and a logical formalization and deduction of the golden rule of ethics.
"Introdução à Lógica", Cezar Mortari - The author of this book is brazilian, so it's written in portuguese, and I don't know if there is an english version of it, but maybe some lusophone is reading this and I hope it will be useful. It's a very technic book, with precise definitions. It contains almost everything that Gensler has, except for deontic, imperative and belief logic and the ethics formalization, and it has a basic arithmethic theory formalized, something about axioms system and other things.
"Introduction to mathematical logic", Elliott Mendelson. - I didn't read this book yet, but i heard about and people say it's a good book to start studying math logic.

Bertrand Russell, An Introduction To Mathematical Philosophy.

The definitive introduction to Peano arithmetic and set theory. In this book you will find (for example) proof that an infinity may be less than some natural number.

Hi,

I'm looking for a good book on QML. Any suggestions welcome.

Beginning Logic, E.J Lemmon

Intro to formal logic.
I enjoyed the exercises.

Introduction to Formal Logic by Peter Smith. Covers the basics, from propositional logic to first order logic. Instead of using natural deduction, it focuses on more mechanical proof techniques (such as tableau method)

How to Prove it by Daniel Velleman. Gives a brief introduction to propositional and first order logic. Doesn't delve much into formal logic; it's geared more towards people who need to learn how to prove math theorems.