[capink]

Two options:

• If the zero length format has a unique extension (I imagine it must have), use the following condition (will assume unique extension is 'abc'):
  
  Template:
  

  {formats}

  Datatype: text
  Comparison: !=
  Condition Value: ABC
  
• If they have no unique extension, you have to tag these books with a unique tag (e.g zero-length), then use the following condition:
  
  Template:
  

  program: 'zero-length' inlist $tags

  Datatype: text
  Comparison: !=
  Condition Value: 1