Not sure what "book" you are talking about. Why would one need a serial number for a book? Is it maybe not a printed book but an encrypted e-book?
Or are you referring to the Photoshop application itself?
In any event, if you felt defrauded, you need to seek redress from the girl that sold you the item. Neither Adobe nor the users of this forum can help you in any way. Sorry.
Some books have additional content available online, but there again, I can't remember any needing a serial number. I have just checked my bookshelf (I have one shelf full of Photoshop and related books) and several still have their CDs intact and unopened, and none of those have a serial number.
But having typed all the above, there is a niggling thought in the back of my mind. Peachpit perhaps?
I keep thinking the book came with a trial for photoshop on the dvd-rom and the buyer might have thought a serial to make it a licensed version came with the purchase.
Even then we can't do anything. The obvious is to clear it up with the seller.