An out of print book is a book which is no longer being printed or reprinted by its publisher. This means that supplies of the book are limited to the stock on hand at bookstores, along with the used copies floating around in general circulation. Many books go out of print as their popularity wanes and people move on to other books, which can sometimes be frustrating for people who are looking for particular books.
There are all sorts of reasons for a book to go out of print. The simplest is usually a response to demand. If a book is no longer popular, the publisher is not going to waste energy reprinting it. A book can also go out of print as a result of a publisher bankruptcy, in which case a dispute over rights may make it difficult for another publisher to print the book.
Many publishers distinguish between a truly out of print book, as in a book which they have no plans to reprint ever again, and a book which is temporarily out of print or out of stock. Publishers may allow books to go temporarily out of print so that they can dedicate their facilities to printing other books, or so that the supply of books will be used up before the publisher prints more. In these cases, people who are patient may be able to obtain a new copy of the book when it goes back into print.
In the case of a truly out of print book, consumers who want a copy of the book must launch an out of print book search. If the book is relatively recent, copies may be available in a new bookstore, and they may also be relatively easy to find at used book stores and thrift shops. Older books may need to be obtained from antiquarian booksellers who specialize in stocking older books, particularly out of print books. Many bookstores will perform searches for a fee, using a database of antiquarian booksellers which requires an annual subscription fee.
Being out of print does not necessarily make a book rare, or even particularly valuable. The determining factors in value are how many books in total were printed, how many copies have survived, and how culturally important the book was. If a single edition of only 5,000 was authorized, the book may be quite rare and valuable, whereas a book which has been printed in the millions should not fetch a terribly high price. The older a book is and the fewer numbers printed, the more valuable it will be, as a general rule.