E-books of just about any type (.mobi for Kindle, PDF, etc) can and are stolen and passed around on peer to peer sharing networks all the time. ISBN or no ISBN - just like music, movies and video games.
As far as plagarism, there always seems to be a writer filing suit because someone has copied their work, but rarely in full. Sometimes book fans will cause an uproar because someone's plot or characters too closely resemble someone else's work.
There is also a huge black market for mass-produced, illegal copies of popular novels in some countries. I think that, when the final Harry Potter book was released, it was actually bootlegged and sold in China before or close to the actual release date, but I could be wrong on the timing,