Yes, he's right, this happened to me and I had to sync my iPad again. If that doesn't work, delete books off iBooks app on iPad and then resynch to iPad. Reset iPad afterwards to clear any problems. Double press home and off switch until you get slide to turn off then wait until shut down and then double click both buttons until apple sign reappears. Don't know why this happens to some of us but there you go!
Sent from my one and only original iPad using iPF