A used book or secondhand book is a book which has been owned before by an owner other than the publisher or retailer, usually by an individual or library.

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Some new book shops also carry used books, and some used book shops also sell new books. Though the original authors or publishers will not benefit financially from the sale of a used book, it helps to keep old books in circulation. Sometimes very old, rare, first edition, antique, or simply out of print books can be found as used books in used book shops.

A reading copy of a book may be well-used, may include highlighting or marginalia, and is suitable for reading, but is not collectible. This is a term used in the used book business, to indicate the lack of collectible value, while claiming that the book is in sufficiently good condition for a purchaser whose interest is primarily in actually reading the book. A reading copy is typically less expensive than a collectible copy.