No, it's not an idiom. We describe a sharp upward angle as "steep". A set of stairs is called a flight. The hope and inspiration are salvaged books. The text is here: http://nicol29ad.tumblr.com/post/149209369977/secret-library?is_related_post=1

In this sentence, 'a steep flight of stairs' is talking about the steep angle of the stairs that lead to the basement. It wouldn't be an idiom, although I think the 'pieces of hope and inspiration' is probably a metaphor for something. Hope this helps :)