Hmmm perhaps this should be an iBooks option.
My assumption is that last page open is/or can be an iCloud function if the book is installed on more than one Apple registered device. Which means that more than one person reading the book simultenously. How then does each person have the book open on the last page they read or is that a bit of iCloud magic.
What is really annoying is if you delete the book off an iPad and then sync it again it always remembers the last page open so deleting the book does reset the book to open at the Introduction. Cheers Ant..