I have 2 really old books i cant find information on them....?

I have 2 really old books i want to know how much they are worth but i cant find information about them the books are a mysterious disappearance by gordon holmes first edition 1905 and isobel by james oliver curwood 1st edition 1913. does anyone know a site where i can go to see how much there worth i would really appreciate it. how much do you think they are worth also. Thanks much for someone who can help me