Based on the work I did in 1985 at Bell Labs as part of my assignment to create documentation requirements for the Acorn Network Control System -- Good documentation should have at least 4 parts. Each particular user persona would use the 4 parts in different ways. Part of the documentation would appeal to potential customers, novice users, intermediate users, users with limited but deep domain expertise, users who previously had fluency with the product but who lost that fluency due to lack of use.
#1 The first in additional to typical table of contents found in each manual there should be a documentation MAP, that combines all of the various documentation and training for a specific product into a visual map; typically this is done with a task orientation. Much like a web page site map, this will allow a potential readers with a wide range of user cases to find the right document or the correct chapter, section, and should include online training, videos, Etc.
#2 A quick reference guide which I think most users would be familiar with. This instruction is typically very linear, and walks at a high level users through the major steps for the most typical cases.
#3 A "cook book" best for coding applications but has broad application to most technologies. Each section of this manual details how to perform a particular task, in it's entirety; e.g., a recipe. Recipes should cover a range of users types (novice, intermediate, expert, or with specific previous domain expertise). A non-coding example would be: Recipe for setting up a mix minus recording using the Behringer Xenyx 1202fx mixer; the ingredients would include all cables, software, Etc. used in the recipe.
#4 A complete and full reference guide. Again typically found in manuals but often (today) the ONLY section of the manual. Each sub-section is a full and complete deep dive on each part, instruction, or option. This is typically used by experts. It can be used by those who are using the cookbook to look for recipe options and substitutions. It can also be used by potential customers to see if a particular use case is supported.