Alright... I caught you off guard didn't I? Although I really do like my classics, my most recent adventure is Lady Chatterley's Lover; however, I couldn't read it as a beach book. Could you?
Percy Jackson is my choice because it is a book series I always fall back onto. A camp that sits on a beach, everyone living together, having their tea outside in the sun and fighting. Perfect!
I'm not specifying a particular book by Rick Riordan though because I can happily pick up any of his series - any of the ten books - and be content in a sun bed, next to the swimming pool. These books helped me discover who I am and I shall forever be grateful to them.