When you hover the cursor over the lever, were you getting the 'lever' description popping up, with the lever itself being highlighted? Left clicking on the lever once should unlock the door east of the book and candles (left clicking again should re-lock it).
If your mouse cursor wasn't a little bit off when you were clicking (or maybe accidental double clicking?), perhaps there was a problem with that lever, where the game was registering the click behind the lever.
