It is very difficult to speak about antique books. As opposed to other objects, there are no fixed parameters to recognize an antique book that is valuable, since antiquity is not the only thing that gives value to the book. If the only thing we had to deal with was the age of the book, we would only need to check the credit and legal page, where we can find the different publishing dates that the book had had. In this case, we will focus on the last one. Some books do not have this page, which we can find at the beginning of the book before the main text, either because they are too old, or because they have little editorial work. However, the age of the book is not very important; in most cases books are not forged, though there are exceptions.
The most important fact for a book to gain value is the difficulty to get it. Generally, antique books with great print out and few copies left are highly valued. Books inaugurating an artistic or cultural movement, or that were revolutionary in the scientific field are wanted as well. Of course books inaugurating a way of thinking were usually banned by the government on duty, and destroyed. That is why they are so valuable -they were revolutionary, inaugurated a cultural process, and few are left.
The difficulty at the moment of buying an antique book is related to the fact that its value is very changeable: there is not one special thing identifying it as an expensive object. For instance, in the case of a piece of furniture, we can tell whether it is original or not. If it is an original piece, we can find out which period belongs to, and calculate its value. Additionally, the materials used to construct it are still either valuable or inexpensive. On the other hand, the value of the book is not determined by its antiquity or the fact that belongs to a certain period. Neither can we estimate its value based on the material, since cardboard and paper are not currently considered valuable.
If you are buying a book to decorate your bookcase, the best thing you can do is to choose the one you like by its subject and esthetic, and if you are buying it to give it as a gift, then the best thing will be to look for a book that is interesting for that person. However, if you wish to buy an «antique», then you should hire an expert in antique books and enter the small world of really valuable books. Yet, we must say these books cost a fortune.
If you are looking for a happy medium, then we recommend that you look for a prestigious bookstore of antique books and trust the salesperson and what he/she has to say about what you are looking for.