Mark Twain

I am trying to find information on a book. The title is "The adventures of Huckleberry Finn (Tom Sawyer's Comrade)by Mark Twain (Sam Clemens)Harper Brothers 1884 copyright 1896 and 1899

Signed saying "This is the authorized edition of all my books. Mark Twain"

I cant come up with a sell price. Could anyone please help me????
Andrea