"An Algebraic Approach to Non-Classical Logics" by Helena Rasiowa was an eye-opener for me. Before that, "A short introduction to intuitionistic logic" by Grego

"An Algebraic Approach to Non-Classical Logics" by Helena Rasiowa was an eye-opener for me. Before that, "A short introduction to intuitionistic logic" by Gregory Mints is a great read which introduced me to Kripke models (and made me see that intuitionism can be understood classically in an intuitive way), but Rasiowa's book just unlocked how I saw logic from then on.