In general, antiques are considered to be 100 years or older. This stems from customs law. For book collectors, however, age is not the primary consideration. A first edition, first printing copy of "Harry Potter and the Philosopher's Stone," for example, sold for $37,000. An 1845 first-edition copy of Edgar Allan Poe's "The Raven," meanwhile, can be had for $400. But money isn't everything. Books have tales to tell, and if you've stumbled on an old volume, you likely want to learn its history.
Look at your book's cover. Modern books as we know them, mass produced in a uniform manner, date to around Civil War times, according to "Book Finds: How to Find, Buy, and Sell Used and Rare Books," by Ian Ellis. Before the mid-19th century, books could be custom bound for wealthy readers in luxe leather, or purchased as serials bound in raw paper. Between the Civil War and the International Copyright Act of 1891, as literacy grew, books were quickly mass produced as "truly ugly, appalling things" to meet the demand, and then became more standardized with cloth covers, Ellis says.
Check the dust cover. If your book has a dust jacket, it's likely no older than the late 19th century, and probably much more recent, because these are fragile. The Victoria and Albert Museum in London, which has a collection of dust jackets, says few survive from before the 1890s.
Open your book, and look at the copyright page on the back of the title page. Modern books' title pages tell you what year the book was published and in which edition. Some include information such as the publisher and city where the book was published. Books published from 1970 on include an International Standard Book Number, ISBN. Books from the 19th century and older might not include a copyright date, though, so that's where it gets interesting.
Delve into research about the author. If your book has no copyright date, check bibliographies of the author's work, suggests "Empty Mirror Magazine of the Arts." You also can research the illustrator, the publisher and references in the text -- if the last year referenced in a history is 1882, the "Empty Mirror" author points out, that's a hint the volume is from around the turn of the century.
Run your fingers across a page. Experts can tell how old a book is by the look and feel of the paper, and its method of printing the text and illustrations. If the book was put together on movable type, dating from Gutenberg and 1450, it will have raised type that you can feel on the page. Eighteenth and early 19th century printers might have used an intaglio process. Modern books use offset printing, in which the letters feel smooth and flat.
Check the paper quality. Wood pulp paper, still used today, dates to about 1850. Rag-based paper, which might have a watermark, and is strong and flexible, could mean the book was printed before 1850. But modern books sometimes use rag-based paper to simulate an antique look, according to "Americana Exchange Monthly," a publication for book collectors.
Once you've looked at your book's cover and what's inside, you can research it further through online booksellers and at your local library.