after I buy the book, a window appears and says "unable to authenticate user" any sugestions?

Go to your account a tthe online Appe store (store.apple.com) and reenter all of your information and be sure one click ordering is on

Before ordering your book preview it using this method - http://support.apple.com/kb/HT1040 - and save the resulting PDF for reference - the delivered book will match it.

if the preview is good try again

LN