I had the same issue with the book showing as paused. Here's the steps I took
On iPhone
-close the iBooks app down if its still open (double click Home and press red minus symbol to close)
On iTunes
-in Books section choose 'Selected Books' and UNTICK the book in question
-Click Apply button which will remove the book via another sync
After the sync has finished then TICK the book in question
-Click Apply which will re-download the book to the iphone
On iPhone
-start up iBooks and the book should download automatically
( DON't click on the Book until the download has finished in case you pause it again ! )
This worked for me and its pretty ugly but I did enjoy my book eventually.
Good luck!