Rare books are an amateur passion of mine. I don’t pretend to be an expert, but I’ve taught myself a lot over my years working at bookstores, libraries, and special collections departments.
The Basics:
OLD RARE BOOKS
Old does not necessarily mean rare. This is a common misconception. If your book is from the 16th or 17th centuries, you may have something. If it is from the 19th century, then your book will have to pass a lot more tests to be considered rare.
Is it a first printing? It can be hard to tell with older books. Look up what date the book was first published, and see if that date matches the date on the title page. If you’re unsure, you can think about having the book appraised. A first printing by a famous author may be worth a lot, depending on the condition. A very old printing that is absolutely gorgeous but is not a first printing may not be nearly as rare (but just as wonderful to own). A book that is not a first printing but is an important printing (there are various reasons this may be; this rule particularly applies to nonfiction, though) may be worth just as much. You’ll need an expert to tell you.
A first printing by an unknown author is not likely to be valued highly… but look the book up. Sometimes there is a good collectors market for obscure titles.
Rare does not necessarily equal valuable. A rare first printing of a major authors’ book is likely to be very valuable… but the true value will depend on the condition the book is in. If you have something good that’s very beat up, you can consider having it restored! If a book is rare in the sense of scarcity, but it is obscure and there is no market for it, it will not be valuable either.
Sets! Many book collectors have beautiful old sets of popular authors, such as Shakespeare or Dickens. The value of your set will depend on a lot of factors: is it leather bound? Is it bound in a particular style, or by a famous binder? What is its condition? Is it a complete set?
MODERN RARE BOOKS
If it is not a first printing, it’s probably not rare, unless there is some other mitigating factor like an interesting typo or particularly important illustrations. You can tell a modern first printing by the number line on the title page. There should usually be a 1. There are exceptions to this, especially if the book is printed by Random House. (First printings and first editions are not the same thing; edition refers to how many times a book has been revised, printing refers to how many times the same edition has been reprinted. You’re looking for a first printing of the first edition; later reprints such as the paperback or a reissued hardcover may also be first printings of their respective editions, but they are not a true first. This type of first printing is known as a First Printing, Thus.)
A first printing is not automatically worth anything. It has to be a first printing of a very important or famous writer or book, and it must have had a small initial print run. Harry Potter and the Philosopher’s Stone had a 5000 book print run in its first edition; a copy of this can sell for many thousands of dollars. Harry Potter and the Deathly Hallows had an initial print run of millions of copies and so a first edition isn’t worth much.
A signed first edition (of a popular or important writer or book) is the best. If it’s a signed first edition of a famous authors’ first book, that’s even better. If it is inscribed to someone, the value lowers. If it is a signed later edition the value lowers as well. For example, one of the more rare books we’ve had at Open Books (my store) was an uninscribed, signed and dated copy of Christopher Hitchens’ first book Cyprus. Again, a signed first printing must be that of either a very important or famous writer or book to be considered rare in the book-collecting sense of the word.
And of course… for every rule, there’s an exception.