Share 'Dating a vintage Washburn acoustic'

Can anyone here recommend any resources or reading materials for accurately determining the age and approximate value of older, pre war Washburn acoustic guitars? The guitar in question, in this case, is a parlor sized Washburn model A 5991. The serial number, stamped on the neck block, is 3991. A woman brought this guitar to me to be fixed up, and she wanted me to find out as much about it as I…