ISBN stands for International Standard Book Number. It's a 10 or 13 digit number used for identification. Every book has its own unique ISBN number. You can find the ISBN number labeled on the back cover of a book, right near the bar code. It's also inside the front cover on the copyright information page.
If you know the correct ISBN of the book you are looking for, it will absolutely be the correct book. If your professor or syllabus tells you the ISBN of a book to buy, search for that ISBN because it will always be right. If you are confused about the exact edition, title, or author of your book, get the ISBN number from your Professor, classmate, or directly from your campus bookstore.
(Search by ISBN without any dashes like this: 0471321682).