Nope. If you have the book you are good.
Did you give money to the town mayor?
Viewing post in Oriel and the Black Knife...v8.1 comments
Yes, both before and after getting the book. Second time got me a Tear Of The Sun. Funny thing is, after saving and loading my game, I tried to enter the cave again, but it still said that the cave is flooded. After that, I tried to pay the mayor again, and he gave the hint again, after which I could enter with no problem.