I think we can approach this several ways. One is to use statistics that are available on the internet such as the percentage of people who own the devices that allow them to read e-books.
If you take these figures and go to the webpage of the US census using their latest population projections you can do some calculations.
Suppose you are writing something that may be of interest to African Americans. With a population of 40 million and 8% e-reader ownership, that is 3.2 million people.
Or suppose you are writing something that may be of interest to people 50-64 years old, of which there are about 58 million in the US. From the table 13% of them own e-readers, so that is 7.5 million people.
Let's assume that about 1 million e-books are sold per month in the US. If you are writing fiction that is a market of 600,000 e-books a month, but if you are writing about religion it is only 40,000 e-books per month.
I don't claim that these approaches are accurate as there are many other variables to take into account. I am just trying to figure out a way to gain insight into the size of the market for the books we write and obtain a ballpark figure.
If you have a method or observation you would like to share or you know of some website or other resource that may be of help, you are welcome to leave a comment.