Gather all the information on the book that you can. Know the author, publisher, publishing dates and country of origin.
Search online auction sites. This is an easy first step to determine how rare the book is and whether there are any legitimate copies for sale.
Get to know your local librarians, antique dealers and used-bookstore dealers. Developing personal relationships with people who see books daily increases your chances of getting an insider tip when your treasure pops up.
Check out local or well-known antique bookstores. Big cities like Chicago and New York have antique bookstores with huge selections.
Sift through the book sections at thrift stores and donation locations like Goodwill. Donations are often made from estates; valuable items can be found with persistence.
Check out antique malls when you are on the road. Taking advantage of vacation travel to walk through little-known antique stores increases your chances of finding an old book.
Focus on the genre or theme of the book and contact Internet forums or clubs that relate to that theme. Post information on the book you're looking for to see whether you can make a connection.
Recruit your friends via social networking sites. Let people know what you are looking for, including the quality and price you seek, on Facebook, MySpace and Twitter.