If I have an ARC that’s either about to expire or about to publish, I’ll try to knock that out. I’m usually good about staying ahead, or at least caught up on those, so I have a little more freedom. Sometimes it’s about genre: I’ll have just read three “literary fiction” titles in a row, and I’ll want a nice YA-Paranormal to blow through, sort of like literary sorbet to cleanse the palate. Other times, I’ll have read consecutive non-fiction titles, and I’ll look for something fictional. Many times, it has to do with length. If I want a book I can plow through, I’ll find a shorter title in my reader. Sometimes, if nothing’s pressing me, I’ll break out a nice fat biography–I have Gerald Clarke’s biography of Truman Capote ready for a rainy weekend.
So basically, there’s neither rhyme nor reason. I just look in the reader, pick a title, and go.