If I could recommend two books to study in the pursuit of becoming a clear thinker, I would start with Peter Kreeft’s Socratic Logic and Fr. William A Wallace, O.P.’s Elements of Philosophy. Socratic Logic is the clearest and far and away the best book on real, Aristotelian and useful logic in print. It should be read, reread, and all the exercises in it completed by anyone who aspires to be a clear reasoning person. The Elements of Philosophy is a compendium, and is a tool for use in studying all of philosophy. Almost everything one would wish to know is presented, but in seminal form and very densely. It is therefore a guide to studying all the topics of philosophy, whether systematically or historically.
There are certainly other exceptional guides and introductions to philosophy, but none seem to be as complete as The Elements. To begin a study of the history of philosophical thought, no series comes more highly rated than that of Fr. Frederick Copleston. I suggest, of course, that one start at the beginning.
This is a slightly adapted outline for the study of Philosophy for Dominicans (and I would say any clear thinker) from Fr. Benedict Ashley, O.P.
Introduction to the Guide
I: Methodological Questions
II Natural Science Questions
III Ethical Questions
IV Metaphysical Questions
The full guide can be found HERE
A great book by Fr. Ashley is The Way Toward Wisdom. It is certainly a difficult book in many ways for those new to philosophy, but a very important and complete book which follows in some ways this outline and gives a big picture of reality, as well as constantly demonstrating how we have lost sight of this “big picture” in our very fragmented system of education, especially at the University level.