This font looks so simple, but for some reason I can't remember it's name...

thanks in advance.

Swiss 721 Bold Condensed  Suggested by Jim.Ramone 

Bryan Lee O'Malley has stated the name of this font on his tumblr:

"The font for all the small captions in both the books & movie is Swiss 721 Bold Condensed. Sometimes in later books I used the regular weight and italics too.”