An indexer must be able to grasp the themes of the book, as well as the subthemes, a task more easily stated than accomplished. Especially in academic books, the themes and subthemes can be interwoven until they are almost impossible to untangle. But untangle, an indexer must.
The secret is to identify the main themes of the book, and to recognize how all else is built upon these themes. Once you successfully identify the main themes, primarily from the Table of Contents, then you have the basic scaffolding for the book.
This is especially vital when indexing academic books, which could be simple and straightforward, but are more likely convoluted into numerous themes that entwine.
A psychology book, for example, might have fairly straightforward subject matter, but once you begin to index, you find that many of the ideas are intertwined and not easily separated. Duplication is tough to index, but if you keep your main themes in mind, you will find a way to attach the subthemes in proper place upon the initial structure.
A history of genocides of the nineteenth century (as I once indexed) is certain to cause endless hours of creating topics and then subtopics, and then rearranging those very topics and subtopics, as connections are made and topics continue to expand. The risk lies in losing sight of the primary scaffolding and building off on tangents that threaten to topple the structure.
If you feel yourself lost in a maelstrom of seemingly unrelated topics and subtopics and cannot find a focus, return to the Table of Contents. Return to the basic topics of your initial structure.
If necessary, imagine stripping all added elements off the structure, laying them to the side until you can begin to replace them in an ordered fashion on the structure.
Hold aside anything that doesn’t fit nicely on the structure, leaving that until the end. Once all other items are placed, you will likely see the perfect placement for what is left, or know that it doesn’t belong.
Trust the integrity of your structure, and you will have a firm foundation for your index.