I like to have two books I'm actively reading at most times, typically one technical-ish book and one nontechnical-ish book. If I need more than two then probably I'm not actually interested enough in one of them and will just stop reading it altogether. I have a lot of books I've read 1% - 50% of and never finished, though coming back to one later isn't totally out of the question, it just goes back from my "reading right now" place to my shelf.