Isaac Asimov donated his correspondence to the Boston University library (at their request in 1964 - they were compiling the correspondence of every author who'd attended Boston University). His brother, younger than Asimov by 9 years, edited some of the correspondence into a book.
Asimov didn't write long letters - indeed, more than half his correspondence with fans took place via postrcard - he saved his energy for writing his articles, stories and books, but there are some interesting letters here.
Here's what Asimov said about why he wrote his non-fiction work:
I write in order to teach and in order to make people feel good (for I am wedded to the theory that learning is the most enduring pleasure.) However, my chief reason for writing is to please myself, because I myself learn by writing.
I start work every day (including Saturdays, Sundays and holidays) at 7.30 am. I stop work every evening at 10:30 pm. The work is continuous except for interruptions which are numerous (telephone, business lunches, social engagements, biological functions, talking to my wife) and which are resented. I work in my apartment and interruptions don't slow me down since at the conclusion of the interruption I begin, full speed, at the point I left off.
(Regarding his non-fiction)
Rightly or wrongly, I rarely wade through primary material. My references are generally a wide variety of encyclopedias, dictionaries, textbooks, etc - in other words, predigested stuff, secondary material.
This would hurt my conscience if I ever pretended to be contributing anything to scholarship, but I don't. I cheerfully admit that I never present anything new. What I have to sell is arrangement and style.
And speaking of hard ecnomic times...
In the 1980s, however, I returned to novels and have turned out seven of them in eight years. The money has just poured in, in a vast flood. Why did I change? I don't need the money. I have nothing to spend it for. My needs and wants are very few.
However, I realized that I was approaching the end of my life. I will leave behind, as survivors, a wife and two children. Consequently, in the 1980s [he wrote this on 31 October 1990] I bade farewell to ease and happiness [of writing nonfiction] and applied myself to laying up financial fat so that my survivors can live through the hard economic times I saw coming as soon as Raagan became President.