Window seat with built-in bookshelves. Could use IKEA: Hemnes daybed kitchen pullouts each), and Billy bookcases each or with uppers each). Total cost with knobs, trim, paint, etc. would be about The skylight is a nice touch too!
Reading Nook with Built in Bookshelves
Reading Nook with Built in Bookshelves(Favorite Spaces Bookshelves)