Initially, I started research for my second book with the intent to write one book. But as the research has gone on, I've come up with ideas for two more books.
Since they all involve the same subject, the presidents, but with different emphases, I'm now doing research for three books, since I don't want to go through those same books a second and third time.
The brain gets a good, strong daily workout in my attempt to keep in mind everything I need while I read the necessary books.