The Associated Press Stylebook and the Chicago Manual of Style are very different guides for two very different groups of people who make their living with the written word.
The information contained in each has a small crossover factor; but, in general, each is a specialized reference work for its intended profession.
The AP Stylebook concerns itself with a much smaller group of writers: those who produce newspaper copy and the writers concerned with public relations and informational news releases. The citing of sources is treated much differently in news media once the final product is produced, and there is very little regarding the citing of sources in the AP Stylebook.
The Chicago Manual of Style is a much larger and detail oriented work primarily because of the breadth of its intended audience: writers of every kind from the latest mystery author to the doctor writing articles for a medical journal. It covers the concerns of editing all of those works and laying out the various works by publishers. Large portions are devoted to citing other sources, reference lists and bibliographies.
The actual rules content and instructions for setting up news articles correctly is surprisingly a small portion of the AP Stylebook. The majority of the book is devoted to a combination of dictionary, thesaurus and encyclopedia covering the most commonly misused or confused places, words and concepts presented in news articles.
It is much more geared toward ensuring that the factual information regarding such things as the correct usage of titles of nobility or the correct meaning of military acronyms are observed, than laying out rules for the actual writing of a news article.
There are certain guidelines regarding construction and formatting of different types of articles but percentage wise, they are a very small portion of the contents.
The Chicago Handbook of Style is much more oriented to the technical aspects of writing and publishing in a correct fashion.
There is an extensive section on punctuation and an even more extensive section of the correct quoting of sources, quotes and references.
Separate sections are included for tables and charts, the inclusion of illustrations and how to properly express mathematics and numbers in written form.
There is a portion on names and terms somewhat similar to that in the AP Stylebook but much smaller and intended more to guide the writer in proper inclusion in sentence structure and usage than factual accuracy.
A similarity between the two works is that both have guidelines to prevent their users from falling prey to having their writing questioned.
In the case of the Chicago Manual this takes the form of comprehensive guidelines for distinguishing between ones original work and quotes or sections attributed to another source or author. This guidance aids the writer in guarding against charges of plagiarism or intellectual property infringement.
The AP Stylebook has a section similar to this but specific to a concern more commonly encountered by news and media professionals: libel.
The guidance regarding libel presented in the AP Stylebook is not intended to be a textbook or comprehensive legal guide but rather a working guide for the writers and editors.
Topics covered in this section include:
The AP Stylebook is very clear that any complex questions of libel and associated topics should be brought to the attention of competent legal advisers and that the Stylebook guidance is not the definitive answer to any given issue.
The AP Stylebook and the Chicago Manual of Style are both written in the same arena but are directed at two different sets of contestants.
In book and article writing, the creator has weeks, months or years to get every detail correct and the Chicago Manual reflects that high level of craftsmanship. In news media, however, deadlines and the need to publish immediately demand a much more rough and ready guide that sets general rules and relies on the individual writer's talent and the editor to make sure the details come out right.