I don't seem to have that problem. Make sure iBooks is up to date. If it is, then try force closing the app and restarting it by double taping the home button, holding the iBooks apps until the red "-" appears. hit the "-", then close multitasking by hitting the home button or touching anyplace on the screen about the multitasking bar. If that doesn't resolve the problem, try delete and reinstall iBooks. If that fails, try hard resetting your iPad by holding down both the home and power buttons until it restarts.