I like to work on a few projects at a time, though not every one of them sees the light of day. The metaphor I often use to describe my writing process is that of cooking, namely that I have a front-burner project, a back-burner project, and just as often a slow-cooker. And at any point one project is just as likely to bump another project off the stove, or at least to another burner.
Take my Yankee book: The Men Who Made the Yankees was a slow-cooker for the better part of two decades. Then all of a sudden I made some unexpected progress and it leap-frogged to the front burner and stayed there until I finished it. That's how it happens. You just never know.