I routinely have 3 books going simultaneously. Right now I have 4 plus 2 war games going as I prepare to teach a Leningrad class in the fall and Russian front class in 2017.
I am afraid I am the greatest villain in too many books at once. Between 20 and 40 is typical for me. They generally stay in the room assigned to them until finished.
I have books at my cabin that stay there, unfinished, until I return once or twice a year. Some books have taken me over five years to finish with some requiring multiple restarts. However, most of my reading is history or political theory. Burke is still Burke even if sat down for two months. Waterloo still looms no matter where you are 200 years ago so it is easy to rejoin the narrative for me.
It is how I have read due to business travel. The books in the trunk or in the suitcase, don’t get unpacked, only the dirty clothes. The books stay there two to four days until I departed again.
My wife reads a single book, regardless of lack of appeal, from start to finish and then pencil dates the front fly page. Only when done does she start another other than cookbooks or reference works.