I'd go w/ a Kindle. You can pick the lightest one, like Stick, or the back lit one (Paperwhite; longer battery life but heavier). A 2-L Sea to Summit SilNylon dry bag will easily fit and keep it waterproof.
FYI... You can easily download books from your library on the Kindle (electronically check them out just like a 'real' book). The Nook requires more hoops to jump through to do the same thing. A librarian who owned a Nook told me this.