Twenty years ago many antique books of well-known authors from the 1700s and 1800s and early 1900s could be purchased for $50 - 200. But that's not the case anymore as first edition books of authors such as Charles Dickens, Mark Twain, Stephan Crene and Jack London have skyrocketed.
Today, the interest level in antique books is high, possibly due to The Antique Road Show. Many people are looking for old books in their grandparent's attic, at estate sales and at garage sales. Once they find old books they want to know what they are worth.
It can be difficult to put a value on an antique book which is generally defined as a book that is a hundred or more years old.
Books from the 1800s and earlier didn't have dust jackets. They are usually worn and torn and stained. And often they have been repaired. If all the pages are intact and the boards and spine are in relatively good condition there may be some value in an antique book.
But many books were reprinted many times and are relatively abundant and common. You'll still want to look for first editions as with modern and hypermodern books. And you'll likely want to get guidance from a book dealer who specializes in antique or rare books to learn the pitfalls and to get an idea where bargains may be found.
One good source of antique and rare books, antique book values and information is Advanced Book Exchange which is the largest used book web site. You can also visit wikipedia for bibliographies of authors. And find information on their backgrounds.
Also, if you have enough interest, time and money there is a Rare Book School at the University of Virginia in Charlottesville. It has been at that location since 1992. It offers five day courses on various topics on rare and old books. There are courses for booksellers, book collectors, librarians, bookbinders and teachers.
The courses have required reading before you attend. Here are some titles that are frequently recommended:
Short History of the Printed Word by Warren Chappell
Five Hundred Years of Printing by H. S. Steinberg
Twelve Centuries of BookBinding, 400-1600 by Paul Needham
The History of Cartography edited by Mark Monmonier
Nabokov's Butterfly & Other Stories of Great Authors and Rare Books by Rick Gekoski
Also, books by Nicolas Basbanes are excellent sources of information on rare and collectible books.
My interest in book collecting goes back about 12 years when my brother-in-law started giving me advice. My brother-in-law has a few hundred antique books that he collected mostly in the 1980s and 1990s on the west coast.
He has shown his books to me from time to time. Most aren't very impressive at first glance until you see the year and the author and know a little about the background of the book.
So if you have an interest in collecting antique books ... read about them, do your homework and talk to the experts (like those at Antiquarian Booksellers' Association of America or ABAA) before making a big purchase.