Since I'm a bookseller by profession, I occasionally receive e-mails and telephone calls asking me to state a value for a certain book or group of books that someone has in his or her possession. This isn't really possible, for numerous reasons. I feel that it would be a disservice to others to state general platitudes about book values - there are, especially in the world of book collecting, numerous exceptions to every rule. The good news is that, while I can't tell you what your book is worth, there are ways of obtaining that information. First off, it's important to understand that a used or rare book, like any other collectible, is worth precisely what someone else is willing to pay for it at a particular point in time.
Value is a personal, and constantly changing, concept.
The same is true in the art world, and actually rare books are a part of the art world. There is no one official catalogue of used and rare books, and while such an immense undertaking might at last be plausible with the advent of computer databases, I would not foresee that even that would be sufficient. Let's take an example. The collector comes across a copy of William Tyler Olcott's Sun Lore of All Ages . These are not, alas, always easy questions to answer.