About ten years ago I read a book trilogy that circled around magicians which conjured demons (or some other form of intelligent extra-planar being) in order to do their bidding.
I remember the series taking place in Central/Eastern Europe during or after the Renaissance but before the Industrial Age. I remember it taking place mostly in Prague (which was the seat of a magical university or some such where conjurers were trained).
Additionally, I recall the conjured creatures having vaguely biblical sounding names.