You can make updates to the content of your book after you have submitted it. There are mixed reports as to how quickly updates propagate through, and some people have reported that submitting an update has caused their currently selling book to disappear from the store; other people have reported that Apple confirmed that submitting an update does not automatically mean the current book is removed from sale while the new version is under review.
It's all a bit of murky area right now. But, if you look at the iTunes Producer Guide and the FAQs, you will find that Apple indeed allow you to make updates to a book.
When you do, there is no way for readers to find out that this happened though.
There are certain parts of the metadata that you cannot change, and you cannot change the preview screenshots once you submit them. Again, read the documentation, it'll be worth your while.
But the short answer is yes, you can make updates.