Some people set the purpose of reading books to make a map to get information efficiently. They grasp the overall map of what kind of knowledge is in which section of the book, and when they need a knowledge they read the section to get information efficiently.
In the database management system, we use a data structure called “index” to reduce the time to search for information. The paper books also a have data structure to reduce the time to search. That is a table of contents and an index.
Books are often read through. However, there are paper documents which usually we do not read through. It is a dictionary. In (1.3.3.1) Even the material has more than 1,000 pages, the table of contents is only 6 pages, I refer to the Python Library Reference. It is similar to a dictionary. When you read this kind of materials, you need to find the section to read efficiently. To make a map is one solution, as I explained in (1.3.3.5) Civil Code Map.
The intelligence to “find information” is replacing by the search engine. I explain it in (Column) Do you need the ability to find information ten years later?. However, often the information found by search does not reach the required quality. It will improve over time, but for the time being, we still need the ability to judge quality. It also is replacing by the collective intelligence mechanism. These include Wikipedia, which uses collaborative editing for better quality, and Stack Overflow, which uses third parties vote for assessing the quality of answers.
To acquire a rough map, reading may not be the most appropriate means. For example, you can read book reviews, ask your friends and ask the author directly. I explained them in (4.2.4) To not read.