Some caveats in searching and buying used rare books are in order. When searching for a book through various online retailers, you will find that a wide variety of prices can exist for the same title. Although price guides do exist for older books, booksellers may price a book depending on the condition of the book, what they paid for it, the edition or what the market will bear. Generally, first edition books that are in mint condition with dust jackets are going to be sold at a higher price than a later edition that is not in mint condition.
Make sure you read the description of the book on the bookseller's web site carefully for the condition of the book. Make sure you buy a book in the type of condition that coincides with your plan for the book. Book collectors stay away from ex-library books, remainders and book club editions because these books are not valuable in terms of money. However, those types of books are fine for the genealogist looking to save some money and who is using the book for research purposes. Books are graded by the following scale: As New, Near Fine, Very Good, Good, Fair, Poor, Ex-Library, Book Club, Binding Copy, and Remainder. For a definition of these conditions, please see the Independent Online Booksellers Association athttp://www.ioba.org/desc.html.
My recommendation is that if you find a book you want but it is out of your budget, try calling used book sellers in your area to see if they have that title. You can also ask the bookseller to search for a book for you and let them know what condition and price you are willing to pay.
Some of the following web sites should help you in searching for that must have read.
Bookfinder.com/ is a search engine that searches through online used booksellers. Using Bookfinder will provide you a way to search several bookselling sites at one time.
Abebooks.com proclaims that it is the world's largest marketplace for books. Through collaboration with over 13,000 booksellers worldwide, they carry new, used, rare and out of print books. On the Abebooks web site you can search for a book by author, title, keyword or ISBN number. You can then sort your results by price, bookseller rating, author or title. In a search I did for Mary Austin's book, Land of Little Rain, I found books priced as low as $1.00 and then a copy priced at $1500.00. These prices reflected the range of copies available, everything from the most recent used paperback copy priced at $1.00 to the more expensive books that were first editions in mint condition.
Alibris.com, has listings for 60 million used, new and out of print items. The Alibris web site includes listings for books, movies, and music. Searches can be done by title, author, and by subject. An advanced search allows you to search by title, author, subject, keyword, ISBN and then you can sort the results by price, condition, title, author, and publication date.
Amazon.com, is a well known retailer for all types of goods. What started out as a place to find books has grown to include pretty much anything you could want. The Amazon umbrella includes used booksellers. From the Amazon homepage you can select the 'books' department and then select 'used books.' From there you can conduct a search on a title or author. The results list will provide you with a list of sellers and prices for the book you are interested in. Like some of the web sites listed above, the Amazon sellers have ratings which can help in making your purchasing decision between several booksellers.
There are several large individual used booksellers that are also worth a look. Powell's Books in Portland, Oregon has a web site atPowells.com. A list of members, including links to their web sites, of the Portland Area Used Booksellers Association can be found at http://www.pauba.org/. One of my favorite Utah haunts, Sam Weller's Zion Bookstore has some of their used books online at Samwellers.com.