Jacques Hadamard, [The Psychology of Invention in the Mathematical Field][1]. 

Description (from the Library Journal):

> Thoughtful and articulate study of the origin of ideas. Role of the
> unconscious in invention; the medium of ideas — do they come to mind
> in words? in pictures? in mathematical terms? Much more. "It is
> essential for the mathematician, and the layman will find it good
> reading."

There is something interesting in page 118:

> At the death of Riemann, a note was found among his papers, saying
> 'These properties of $\zeta(s)$ (the function in question) are deduced from
> an expression of it which, however, I did not succeed in simplifying
> enough to publish it." We still have not the slightest idea of what
> the expression could be.


  [1]: https://www.amazon.com/Psychology-Invention-Mathematical-Field/dp/0486201074/ref=sr_1_3?ie=UTF8&qid=1523527168&sr=8-3&keywords=hadamard%20psychology