The oldest cookbooks I have are ones my parents gave me when I got married nearly 30 years ago. The ones that you all have from the early part of the century sound fascinating. (probaby worth something too.) Even the old Better Homes and Gardens cookbook that my mother gave me seems more old fashioned than any of my other cvookbooks. More basic stuff. My daughter has an American Girls cookbook that corresponds to her Molly doll and the recipes in that are from the 40's. So different.
I love to read old menus too. There's a restaurant near here that has it's old menus from the 20's hanging up. Not just the prices that are really different. The menu is nothing like what you would see today.