The Art of Thought, originally published in 1926, was the first work to give a clear, inspiring, and authoritative exposition of the process of reasoning, in language adapted to the general reader as well as the student of psychology. It established itself as a standard guide to the mechanism of thought, and its message remains of essential value to a world confronted with problems of increasing complexity.
Graham Wallas was an English socialist, social psychologist, educationalist, a leader of the Fabian Society and a co-founder of the London School of Economics. Wikipedia
This book is one of the most cited references pertaining to the thought process and the emergence of creative ideas. Wallas establishes the framework of Preparation, Incubation, Illumination, and Validation.