Used books typically become available on the market when they are sold or given to a second-hand shop or used bookstore; they are usually sold for about half or three-quarters the price of what they would cost new, though rare books and others still in demand or hard to obtain might sell for more than this.
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.