This 1949 book by the psychologist Donald O. Hebb is where the author introduces his theory about the neural bases of learning, which is now commonly known as "Hebb's postulate".
I really liked this book. It took me forever to read, and was somewhat challenging in the beginning, but I now have a much fuller understanding of Hebbian theory, and its applications to emotion, attention, intelligence, and learning. Reading this, I was even more aware of how ahead of his time he really was. I'm really glad that I stuck it out!