If you've ever flipped to the back of a nonfiction book hunting for that one statistic — the one you knew was in there, somewhere around chapter seven — you already understand what a book index does. You just may not have thought about how it got there.
For first-time nonfiction authors, the index is often the most mysterious part of the production process. It's not editing. It's not formatting. It shows up at the back of finished books looking like it generated itself. And once your publisher or production checklist mentions "indexing," the natural first question is: what is actually supposed to go in there?
This is a plain-English answer to that question.
What a Book Index Is (And What It Isn't)
Let's clear up three common misconceptions right away.
A book index is not a table of contents. Your table of contents lists chapters and sections in the order they appear. An index is organised alphabetically and points to specific concepts, names, and terms wherever they appear throughout the book — across chapters, not just within them.
A book index is not a glossary. A glossary defines terms. An index locates them. Your glossary explains what "cognitive load" means; your index tells the reader that "cognitive load" is discussed on pages 14, 47, 89, 92, and 178.
A book index is not a keyword list. This is where a lot of DIY attempts go wrong. A keyword list extracts prominent words. A book index organises ideas in a way that reflects how a reader actually thinks when they're looking for something specific.
What a book index is: a reader navigation system. It's the answer to "I know this book covers X — where exactly is it?" The better the index, the more useful the book becomes as a long-term reference rather than a one-time read.
The Three Building Blocks of Every Index Entry
Professional indexers — and the Chicago Manual of Style, which sets the standard for most trade and academic publishing — recognise three fundamental components of a book index.
Main entries are the primary terms: concepts, proper names, organisations, frameworks, places. Think "leadership," "climate change," "Roosevelt, Theodore," or "agile methodology." These are the headings under which all the relevant page references will cluster.
Subentries are what separate a useful index from a flat one. Rather than listing "leadership, 14, 47, 89, 121, 178" — which tells the reader nothing about which aspect of leadership appears on each page — a well-crafted index breaks that concept into meaningful subdivisions:
leadership
— in crisis situations, 47, 89
— building vs. managing teams, 121
— transformational approaches, 178
— common mistakes, 14
Subentries are editorial judgment in action. They require someone — human or purpose-built AI — to actually understand what the text is saying, not just where the word appears.
Cross-references connect related terms that a reader might search under different headings. There are two kinds: See (when a term redirects entirely to another entry — "Roosevelt, Theodore. See presidents, U.S.") and See also (when a term stands on its own but has meaningfully related entries — "leadership. See also management styles").
These three elements — main entry, subentries, cross-references — are what publishers mean when they specify a "professional-quality" index.
What Terms Belong in a Book Index
For first-time authors building or reviewing an index, the question of what to include is often more confusing than the mechanics.
Include:
- Key concepts, ideas, and arguments central to your book's purpose
- Proper names — people, organisations, places, studies, laws
- Recurring frameworks or models you introduce and name
- Terms your target reader would plausibly search for
- Case studies or examples, indexed under both their subject and their topic (a case study on Apple's product strategy should appear under "Apple Inc." and under "product strategy")
Exclude:
- Function words: prepositions, conjunctions, articles ("of," "and," "the")
- Passing mentions — a word that appears once in a transitional sentence doesn't earn an index entry
- Overly generic terms that would generate unhelpfully long page ranges ("book," "author," "idea")
- Chapter titles, which belong in your table of contents, not your index
A useful benchmark: Penn State University Press indexing guidelines suggest that a well-indexed nonfiction book should produce roughly five to six index references per page of text. For a 200-page business book, that's approximately 1,000–1,200 individual references across all entries.
How Genre Changes What You Index
Genre shapes what a useful index looks like — significantly.
Business and self-help books benefit from indexes heavy on frameworks, actionable concepts, and named methodologies. Readers return to these books as references long after the first read. Your index should help them find the specific tool or principle they remember absorbing.
Cookbooks present a layered challenge: you're indexing recipes, ingredients, techniques, and equipment simultaneously. A recipe for Lemon Herb Roast Chicken might legitimately appear under the dish name, under "chicken," under "roasting," and under "lemon" — depending on what a reader might search for.
Memoirs and narrative nonfiction index more lightly than academic works, focusing on proper names, locations, and key events rather than abstract concepts. Many memoir publishers don't require a formal index at all, and a light touch often serves this genre best.
Academic and reference books carry the heaviest indexing expectations. Dense terminology, cross-disciplinary concepts, and reader populations who use these books repeatedly as working references mean indexes that are correspondingly deep and precise. Professional indexers charge $3–$6 per indexable page for standard nonfiction — and up to $8–$12 per page for certified medical or legal indexing, where specialised subject expertise is required.
For print books, index entries reference specific page numbers — which is precisely why indexing always happens last, after the final typeset PDF is locked and no further layout changes can occur. A single paragraph reflow can shift page breaks across an entire chapter. Build your index from anything other than the final file, and you'll rebuild it later.
For digital-only eBooks, traditional page-referenced indexes rarely translate. Most eReaders use location markers or percentages rather than fixed page numbers. Many ebook publishers skip the back-of-book index entirely and rely on the device's built-in search function.
Hybrid releases — print plus ebook — typically include the full index only in the print edition, with a note in the ebook directing readers to the print version for reference.
The Most Common Mistakes First-Time Authors Make
Indexing too early. The index must be built from your final typeset PDF. If you index a Word document or an earlier layout draft, every subsequent page shift will make those references wrong. Wait until layout is completely locked.
Indexing from the author's vocabulary. Authors naturally notice their own phrasing and terminology. A good index, however, is built for the reader's vocabulary — not the author's. "Brand identity" and "how we want customers to perceive us" may express the same idea in your manuscript. The index entry should use the term readers will actually search for.
Treating it like a word search. A word appearing on a page is not automatically an index-worthy entry. Indexing is editorial work. It requires judgment about what's meaningful, what's searchable, and what will genuinely help a reader navigate the book on a return visit.
If you're a nonfiction author approaching production, the index doesn't have to be the mysterious final step. Understanding what goes into one — main entries, subentries, cross-references, genre-appropriate depth — is the first step to getting it done without the confusion or the sticker shock.
Onomastic was built for exactly this production stage: upload your final typeset PDF, review the extracted entities, and get a CMOS-compliant index with accurate page references, meaningful subentries, and properly reasoned cross-references — in under two hours. See what your index looks like before committing to any approach.
