On a book, the ISBN can be found inside on the same page that lists the publishing information and copyright, or on the outside back cover of the book or the book jacket.
For textbooks in which several revisions or editions have been published, the ISBN code can be extremely important. At Bookfinder.com you can search for books with an ISBN code, but also by author and title. Their advanced options will allow you to select the language (Dutch, English, French, German, Italian, or Spanish) as well as new or used books, 1st editions or out-of-print, and a price range if so desired.
AMAZON.com is a great resource for finding books by ISBN or for finding the ISBN for multiple editions and formats of the same title. NOTE: Amazon has created their own code known as the ASIN (Amazon Standard Identification Number). If a book has an ISBN, the ASIN will be the same, but the ASIN comes in especially handy for books published prior to the late 1960s that were not automatically assigned an ISBN. There is some controversy surrounding Amazon's ASIN - some say it is a proprietary product identifier, but I have a feeling it will become part of the International Standards in years to come.