Enter an ISBN to validate it and find the book it identifies in the Open Library catalogue.
An ISBN identifies one edition of one book from one publisher, hardback, paperback and e-book editions each carry different numbers. The lookup first validates the number locally: ISBN-13s use a weighted check digit like barcodes, and ISBN-10s use a modulo-11 check where the final digit can be the letter X (standing for 10). A valid check digit proves the number was transcribed correctly. It then queries Open Library, the Internet Archive's open catalogue of tens of millions of editions, for the bibliographic record: title, authors, publisher, publication date and page count where catalogued.
Coverage is deep for English-language and academic publishing and thinner for very new, self-published or regional titles. 'Not catalogued' means the open catalogue lacks the edition, not that the ISBN is wrong, the check-digit result separates those two cases cleanly.
Can an ISBN-10 be converted to ISBN-13? Yes, prefix 978 and recompute the check digit. The results show the equivalent form when a 10-digit number is entered. (The reverse only works for 978-prefixed ISBN-13s.)
Why do two copies of the same book have different ISBNs? Different editions are different products: publisher, format, and sometimes content differ. Libraries and booksellers need edition-level precision, so each gets its own number.
Does a valid check digit mean the book exists? It means the number is structurally sound. Assignment to a real edition is what the catalogue lookup checks.