Antique bottle collecting is an avid hobby among many people around the world. Regular, cylindrical blue bottles are fairly common, and found in many collections, but bottles in unique shapes, like squares or octagons, are uncommon and more valuable. A manufacturer's mark, name, date or pattern embossed on the bottle increases its value even more, and small blue ink bottles are the most valuable of all. Comparing your bottle to pictures on the Internet and in books can help you identify and appraise it, but knowing what a bottle was originally made for is the best way to calculate its worth.
Visit the Antique Bottles website and scroll down the page. The site has pictures of blue glass bottles, their names and how rare they are. You can compare your bottle to the pictures for identification. Pay special attention to the bottle's size, shape and colour intensity.
Visit your public library and search for a book on bottle collecting. A reference librarian can help you locate a guide to antiques in general or bottles specifically. Compare your bottle to those in the guide to see if you can find a match. The library may also have information on any bottle collecting groups in your area.
Visit your local book store and ask if they have a book on bottle collecting. You may find more resources at a book store than at the library. Try one of the general antique guides as well as specific bottle guides.
Some antique bottle collecting books you might want to consult: The Official Price Guide to Bottles by Jim Megura Warman's Bottles Field Guide: Values and Identification by Michael Polak Kovels' Bottles Price List: 13th Edition by Terry Kovel and Ralph Kovel
Always get more than one estimate before selling or auctioning your bottle.