Direct answer: The most reliable sources for an out-of-print book, in order of speed, are AbeBooks (a marketplace built specifically for rare and used books), Biblio.com, WorldCat (to find a library copy), and Open Library's digital scans for titles never reprinted.
Why "out of print" doesn't mean "unavailable"
A book going out of print only means the publisher has stopped producing new copies — it doesn't erase the copies already in circulation. Millions of out-of-print titles are actively bought and sold through used-book marketplaces every year.
Best marketplaces for out-of-print searches
AbeBooks aggregates independent rare-book sellers worldwide and is generally the strongest first stop for anything printed before the 1990s. Biblio.com focuses on independent bookstores and often surfaces different inventory than AbeBooks. Both let you filter by condition, edition, and price.
When no seller has a copy
If a title truly can't be found for sale, check WorldCat for a library holding, then request interlibrary loan through your local library. For older, culturally significant works, Open Library or HathiTrust sometimes hold a scanned copy, particularly for academic or historical texts.
A note on print-on-demand reprints
Some out-of-print classics have been reissued through print-on-demand publishers. These are legitimate but can vary wildly in formatting quality — check reviews before buying a POD reprint over an original used copy.
Are out-of-print books more expensive?
Sometimes, especially for sought-after or illustrated editions, but many out-of-print paperbacks sell for a few dollars simply because demand is low.
How do I know if a book is truly out of print?
Check the publisher's current catalog or a retailer listing — if no new copies are available anywhere and the publisher lists it as unavailable, it's out of print.
Can libraries still get me an out-of-print book?
Often yes, through interlibrary loan, since libraries frequently hold copies purchased decades before a title went out of print.