Furthest page read refers to the highest page location across ALL of your registered Kindle devices and Kindle apps on an account.
So you need to make sure that in every single registered Kindle device (kindle for pc, kindles, kindle for iphone etc) that you open up the book, go right back to the Cover, Beginning or TOC etc (Menu / Go To in a Kindle device) then come out of the book and back to the Home screen and do a Sync.
You need to repeat this process for every single device.
And then you should be able to re-read again from the start with no problems.
You can also call Kindle Customer Service and get them to reset the book for you back to the start.