and then enter the information that you have for the book and a list of books that are currently for sale will pop up. See if there is a book that is of the same description at the one that you have - same date, publisher etc and you will get an idea of the value.
I have found that amazon 'sell your stuff' option is handy for valuing books. Put the book ISBN number in and the condition and it will show you what other people are charging for similar.