ValueError: cannot switch from manual field specification to automatic field numbering

return "{0} by {1} on {}".format(self.title, self.author, self.press)

that doesn't work. If you specify positions, you have to do it through the end:

return "{0} by {1} on {2}".format(self.title, self.author, self.press)

In your case, best is to leave python treat that automatically:

return "{} by {} on {}".format(self.title, self.author, self.press)