Assuming it is a paper notebook, the determining factor is how soaked the pages are. If it is too wet, they may start to meld together making the notebook unusable.

The best thing to do is to actually heat up the book. I've cooked mine in the oven at very low heat, which allows it to dry out fast. My dad does a variant of the rice method for wet books where he fills a bag with rice and then places it in the sun. However, if the notebook is too wet and the pages are sticking together, doing either of these will instead turn your notebook into a solid block of wood. Instead, your best course of action is to try and fan out the pages by individually peeling them apart, then putting the splayed-open notebook somewhere moderately warm where it can slowly dry out under your careful observation.