Try reading a good book on the subject, such as The Man Who Broke Purple, the biography of America's top code breaker, William Friedman (Little, Brown & Company, 1977). His wife was also a top code breaker. In the book, it describes a form of encryption used back in the rum-runner days where all parties had copies of the same book. The encryption involved transmitting a series of numbers for page number and word count on the page. In order to break the code, you'd need to know the exact volume and edition of the book used. An almanac would be a good choice because of the references and maps included. Back in the days described in the book I referenced, the code book was usually some obscure title, but an almanac would work quite well.

Mrs. Friedman actually broke this style of encryption on at least one occassion, but success in doing so would depend on having a good volume of messages to work with along with some understanding of the message context.