Many genius and near genius people have studied thinking and creativity. Another example is the work of the mathematician Jacques S. Hadamard.
In this post from Farham Street, "Einstein on The Essential Feature of Productive Thought", the correspondence between Einstein and Hadamard is discussed. Hadamand went on to publish "An Essay on the Psychology of Invention in the Mathematical Field".
This quote of Einstein from the correspondence is well known but still interesting.
"The words or the language, as they are written or spoken, do not seem to play any role in my mechanism of thought. The psychical entities which seem to serve as elements in thought are certain signs and more or less clear images which can be “voluntarily” reproduced and combined."