In addition to collecting the cookbooks themselves, it's also fun to find the original mail-in offers that match the books.
The original offers were usually found on the product packages and in magazine ads.
The offers on the packages sometimes turn up in collections of old recipe clippings, cut off the box or bag right along with the recipes.
Magazines such as Century, Scribners, The American Family, Woman's Home Companion, McCall's, and Good Housekeeping are just a few of those that contained food advertisements in each issue.
The booklets were often free or obtained for a minimal charge. Sometimes stamps could be used for payment rather than coins, a common practice in the past.
In a 1900 advertisement for Knox's Gelatine in Century Magazine, Chas. B. Knox, known for his unusual marketing methods, offered a copy of Dainty Desserts for Dainty People if one would mail him the name of their grocer. If one didn't wish to give up the grocer's name, he would accept a two-cent stamp instead.
An offer for the New Royal Cookbook from a 1923 Good Housekeeping ad:
An offer for a free recipe book from Brer Rabbit Molasses from a 1935 Sunset magazine:
Promotional cookbook offers are still as abundant today as they were in the past. The individual manufacturer websites usually display their current promotions. Another easy way to find current offers is to Google the phrase "free recipe book".