This book meticulously examines over one hundred documents of research notes by Albert Einstein, many of which were previously unidentified, held in the archives of The Hebrew University of Jerusalem and the Einstein Papers Project at Caltech. Focused on Einstein's quest for a five-dimensional unified field theory of gravitation and electromagnetism, the analysis provides unique insights into his mathematical skills, thinking, and modus operandi. This academic exploration also investigates the role of mathematics in Einstein's theorizing with a special focus on projective geometry and delta functions.