An index is not a list of words in a book. This is the single thing everyone gets wrong about the job, including me for the first two years of doing it.
What the work is
A search function finds words. An index finds subjects, which is a different thing, because a book can discuss something for eleven pages without ever naming it. My job is to read four hundred pages and produce a map of what is actually in there, including the things the author did not label.
That means deciding what counts as a topic, at what level of granularity, and — the hard part — what a reader arriving from outside would call it. The author calls it “administrative rationalisation”. The reader is looking for “bureaucracy”. Both need to be in there and one of them needs to point at the other.
What it did to my reading
Indexing forces you to hold the whole shape of an argument at once. You cannot index chapter four properly without knowing that chapter nine returns to it, so you are constantly reading forward and backward and building a structure that the linear text does not hand you.
After eight years of this I cannot read a non-fiction book any other way. I notice, involuntarily, when a book’s stated structure and actual structure disagree — when the chapter headings promise one argument and the pages deliver another. It is a slightly ruinous skill. A great many books are less organised than they appear and now I can see it.
The compensation
It made me a vastly more generous reader of the good ones. When a book is genuinely well built — when every return is deliberate and the structure holds under indexing — I can now see the work in it, and it is enormous, and almost invisible to a reader going front to back once.
Read a book’s index sometime, before the book. It will tell you within two minutes whether the author knew what they were writing about.