The first major source I would go to is the Study of Mathematically Precocious Youth (SMPY). It is a major longitudinal study following several cohorts of mathematically talented individuals. There are many publications regarding their interests, talents, life paths, achievements, et cetera. The study is headed by Camilla Benbow and David Lubinski.
If you could only read one article describing their psychology, I would refer to Lubinski's review article of 100 years of findings.
Lubinski (2016). From Terman to Today: A Century of Findings on Intellectual Precocity
However, there are many interesting publications from the study, and if you're interested I recommend further looking at their publications.
Historically, neuroscience as a field has suffered a lot from small sample size studies (Button, Ioannidis, et al (2013)), and only in recent years do we get really large scale studies. I personally don't know of any findings that compare mathematicians with non-mathematicians that have sufficient power.
The closest thing I know of to what you're asking, is Richard Haier's book "The Neuroscience of Intelligence" which summarizes what we know and don't know about "intelligent" brains. He emphasizes that no single study is definitive, and that no story about the brain is simple. I recognize that just "intelligence" is not precisely what you asked, but I still think you would find it relevant to what you're curious about.