Just purchased the PMBOK 5th edition (kindle) and enjoying the read very much!! 

A couple of things that make the read enjoyable. 

1. The XRAY feature is awesome as it will show you the bones of the book

2. Popular highlights are availble so you get to see key items highlighted by other PMP aspirants

3. I don't have to carry this book around with me everywhere I go.  I can review my notes on my phone, my computer and my iPad/tablet.