Found "Danielle's Book" in the subway on the floor. So what's it for? You can't read it, and no particular quest requests it. I could sell it, but it seems unique for some reason, like it wasn't suppose to be there.

According to the wiki, http://fallout.wikia.com/wiki/Danielle%27s_Book is not used for anything, but may be linked to the Danielle Faye mentioned in Germantown Police HQ.