There really isn't much to know about the constellations. They are just patterns in the sky people gave names to. At some point every point in the sky was defined to be part of a constellation, thats why there are a bunch of silly, dim constellations. As far as observational astronomy books, I really like
http://www.amazon.com/Backyard-Astro...tt_at_ep_dpi_2 . Although I have an earlier edition, that book taught me everything I needed to know about recreational astronomy. And its definately accessible, I think I read it cover to cover when I was only about 8-10 years old.
Note that this is a book about recreational astronomy. If you really mean it when you say you just want to know the stories behind the constellations, integral8850 is right on. Burnham's Celestial Handbooks are awesome, especially for that. But, I can't see myself ever reading them cover to cover.