erm no worst book really. their all good, the worst movie was PoA. the director and writer changed the entire plot IE: in the book harry and hermione do NOT take a rollercoaster ride from the womping willow, dementors do NOT kill a person like they nearly did siruis and harry. snape did NOT protect harry ron and hermione from the Werewolf Lupin(he was suposed to be unconcious) and one more major thing wrong with that move was the effects of the time turner. (theres so much more wrong with that film)
the base storyline was there:
a prisoner escapes from the wizard jail azkaban, suposedly after harry. harry learns about his father's childhood, and friends.