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I've been startled by the number of books that have incorrect ISBN's. Some don't even validate as checksums (mostly true of small, hobbyist publishers). Other times the IBSN is for the wrong book. When the cover ISBN is incorrect the one on the title page may be right.
If you need the true ISBN the easiest thing is to go to Amazon, enter the author and title. (It may be wrong on Amazon but this has been less common for me.) This is handy if you have a copy of a book before ISBN's were routinely put on books. Many publishers, university presses most often, assigned ISBN's later.
If you are pricing books for sale you need to be wary. A few publishers don't change their ISBN with each new edition. That first edition psychoanalysis manual you have isn't really bringing as much money as you might think. It is the fourth edition that is bringing $40. The earlier editions are so unwanted nobody is bothering to catalog them.