In a perfect world a book with supplemental material for a given game system will have all of the material needed by every GM and player. A book like that would resemble an encyclopedia in the number of volumes and would not be cheap. Which leaves us with our imperfect world and the imperfect supplements that get published, lacking in many different dimensions.
I would like to present my opinion of the sort of topics which should be included in supplements and a short justification for them.
The structure of a supplement revolves around what I call the “Three Pillars”.
The first is the Story part of the book. This is the part of the supplement which gives background information for the topic of the supplement and any available insight into the social structures or daily life as is applicable. It serves the purpose of presenting narration material to the GM and players. It presents story hooks. It is there to answer the questions of “Why” and “Wherefore” when it comes to the supplemental material. It gives justifications. Examples abound; Why do the elder gods want cults instead of going mainstream? How do they create their cults and what sort of people are attracted to them?
The second pillar is the the “How-To” part of the supplement. This section revolves around the numbers and statistics. It gives lists of spells or talents and the like. It gives specific instruction on the “How” of how it all works in the game world. It has any new rules, changes to the old rules, and any reconciliation necessary between the two. If possible, it gives examples of the sorts of common encounters that the GM should now be able to build and how to make them tougher or easier. Idealy, it has all sorts of instruction on how to build from scratch encounters for your players based on the new material. This gives all GMs an insight into the design of the rules so as to be able to give their players the best encounter suited to them.
The third pillar is the “Instant-Go” part of the supplement. It should not be confused with an included adventure or module. This is the part of the supplement where an encounter has been written and is ready to drop in any given adventure. It does not serve the purpose as an example of the material in the supplement but it is the new or changed rules prepared and executed. This gives a busy GM a place to go when they do not have the time needed to create their own encounter with the presented material.
The proportion of each of these sections in a given supplement is not a hard and fast thing. Some supplemental materal, such as a world book, is there to introduce GMs and players to a setting more than to new or changed rules. In such cases, there is going to be far more of the Story Pillar than of the other two. But by no means should the other two be dismissed or ignored. If anything in such situations there should be more attention paid by the writers and publishers to make sure that what How-To and Instant-Go writing is included should be of the highest quality and usefulness. Other hand supplemental material like a bestiary is going to require much of the page be given to the second and third pillars. Again just because it is that sort of supplement does not excuse it from not using the best story possible.