If you say something “jumped the shark,” you know exactly what it means, even if you don’t know its origin (for the record, it was an episode of Happy Days where Fonzi went water-skiing, literally jumping a shark, and that episode was the signpost for the ultimate downfall of the show’s quality).
But is there a bookish equivalent? For example, you can say a series ran off its course in recent years, or the author hasn’t lived up to the earlier books’ promise, but there’s no pithy way to say—in literary language—that a series or author has faltered.
And that is an ironic situation we should change. In comments, please give your cleverly witty bookish suggestions for how we should refer to authors or books that have lost their edge.