4. An Investigation of the Laws of Thought
A classic that has to be included but not as thought provoking philosophically, especially because it is so foundational to set theory and computer science.
What is interesting is its relation to the subject-predicate theory and Russell's critique of it later. Boole makes great progress in the direction of Russel which treated logic more like math via set theory.
Author: George Boole
Original date: 1854