My recommendation for an ".....and then they basically did nothing useful or helpful ever again." book was going to be John Chadwick's The Decipherment of Linear B, which I always thought was about how an eccentric genius British spy channeled his spy skills after the war into the incredibly useful task of translating ancient Cretan artifacts.... but wikipedia says that there's no evidence that Michael Ventris was actually ever really a spy, so instead I'm feeling like a great myth has collapsed on me right now.

It's still a great book, though.