What counts as a 'breach of the skin' for infections(like rabies) that spread via wounds?
I believe that your books refer to 'broken' not necessarily meaning broken to the naked eye, but as in any broken epidermis. 

The outer layer of your skin that protects you against infection is the epidermis. It's a water-tight layer mostly composed of keratinocytes joined together by adherens junctions. Logically, even if you can't see a breach in your skin (epidermis), there may still be one. While the chance of getting an infection from a non-visible break is less than the chance of infection from a visible scratch, it's still possible.