Also, here's a site that lists "rare book values": Fadedgiant Guide.
Professional Book Appraiser
Of course, the most efficient way for you to find out the true value of your book(s) is to have a professional book appraiser or bookseller appraise your collection. As you are probably well aware, the value of the your book(s) or manuscripts depends on the condition of your collection, market trends, and many other factors; so a professional appraisal is important--whether you are planning to sell the book(s) or continue collecting books of the same type. You can search for a professionals--by specialization--here: Antiquarian Booksellers. You can also search for an appraiser here: Books Appraised.