Recently I’ve been reading Graham Wallas’s 1926 book The Art of Thought and his other works, and am now in the process of working it into Rest. Wallas is one of those great characters whose life had all kinds of interesting corners and twists, and it’s easy for me to get side-tracked while researching someone like that. (And I’ve spent a lot of my scholarly life sidetracked, because British intellectual history and history of science is rich with such types.)
The Art of Thought is best-known for advancing the four-stage model of thinking and discovery (Wallas never uses the word “creativity”) that starts with Preparation (always capitalized), followed by Incubation, then Illumination, and finally Verification.
This is a model you see invoked everywhere, but the rest of the book is largely forgotten: it’s only recently been reprinted, after many years of languishing on second-hand shelves and library stacks. But the whole book is quite interesting.
And though in his day he was much more famous for his work on politics and contemporary psychology, it turns out that “the art of thought” is something Wallas spent a long time thinking about. This is from his 1914 The Great Society: A Psychological Analysis:
Is there an art of thought? Modern Psychology, with its insistence upon the essential identity and subconscious action of Memory, Imagination, and Reasoning, might seem to answer, No. But though we cannot control the movement of Thought, we can control a) the material circumstances necessary for Thought; b) the mental attitudes which are favorable or unfavourable to Thought; c) our relation to the subject-matter of Thought. (ix)
Five hundred years ago no one would have had any hesitation in answering, “Yes, such an art of Thought exists; its name is Logic; it was invented by / Aristotle; and it is the most important element in the curriculum of the schools and Universities.” This belief in formal Aristotelian Logic as an art of thought dies hard. [176-177]
Nowadays, the majority, perhaps, of educated men, if asked whether an art of Thought exists, would answer, “No: thoughts come of themselves by a process of whose character we are ignorant, and of which we are always unconscious at the time…. All that we know is that some men think abundantly and consecutively, and other men scantily and at haphazard. 
Anyway, it turns out that looking into Wallas’ life, and his other books, pays off.