In 1939 James Webb Young, one of the original Mad Men and soon to be Advertising Man of the Year, published this classic that is still read by professionals today. Young was a driving force behind the creation of the modern advertising industry, and is one of advertising's most honored educators and practitioners. His treatise on brainstorming is the best I've read and one that I use in my university advertising classes to talk about the design process.
The key principle of Young's thesis is that an effective new idea is a synthesis of two old ideas, and that the ability to generate new meaning depends on your ability to see associations between the seemingly unconnected. He also divides the thinking process for generating new ideas into four steps outlined in the infographic below.
Erratum: The downloadable document has one grievous error made by the typist, in that it gives the author's name as James Wood Young. James Wood Young is a British actor. The ad man and author of this book is James Webb Young.