Under “I”, in the index of one of his books, Douglas Hofstadter (of Gödel, Escher, Bach fame) included an entry for “index, challenges of, 598; as revelatory of book’s nature, 598; typo in, 631; as work of art, 598”. As might be said these days: preach! Indexes are challenging to produce; they are revelatory of a book’s nature; and the best ones are works of art. And, as Hofstadter ruefully if wittily recognised (including under “T” “typo in index, 633” in a book that ended on page 632) they sometimes contain typos. But not, you’d hope, those produced by professionals.

Today, the Society of Indexers – the industry body for those professionals (for which, full disclosure, I have the honour to be honorary president) – turns 60 years old. It celebrates its “anniversary, diamond”. “What?” you ask. “Who?” you wonder. No surprise. Indexers are like badgers: they are seldom sighted in the wild, they do their work in the darkness, and when you see one it’s usually because they’ve been run over by an 18-wheeler.

But now there is the chance to give them a moment of recognition, Springwatch-style. A few weeks ago, one of our society’s members proposed an uncharacteristically immodest idea: why not make 30 March National Indexers Day? Or National Indexer’s Day. Or National Indexers’ Day … It was a bit fraught, to be honest. We’ve settled on National Indexing Day for the good reason that, as the festival’s proposer Ruth Ellis pointed out, “I’d prefer to avoid anything with potential possessive apostrophes as that only leads to pedants debating it on social media.”

Indexers are nerdy like that. Not for nothing is the Society of Indexers closely allied with SfEP, aka the Society for Editors and Proofreaders. And though recruitment can be hard – publishers are getting ever stingier, fees ever lower, lead times ever shorter, proper understanding of the profession ever harder to come by (nobody seems to realise that even a seemingly trivial repagination can leave the indexer having to start from scratch) – this is a body of professionals that is long overdue proper recognition.

It would be a cliche to say that indexers are the unsung heroes of the publishing world. But unsung they generally are: no indexer usually expects or receives credit by name in books where everyone from the font designer to the snapper of the author photograph tends to get a solemn shout-out. And heroes they are, too: the index is, in any nonfiction book, more useful than almost anything else in the apparatus. It is a map of the text; a cunningly devised series of magical shortcuts that can in the good case save a scholar many hours of work, and in the bad one save a bookshop-browsing cabinet minister from having to buy a former colleague’s memoirs.

A good index is, as Harold Macmillan wrote when inaugurating the society 60 years ago, “much more than a guide to the contents of a book. It can often give a far clearer glimpse of its spirit than the blurb-writers or critics are able to do”. Indexers specialise: some are experts in cookbooks; others labour with vital punctiliousness in the thickets of the legal profession; others will know chemistry, maths, or the politics of sub-Saharan Africa. To index a book you need to understand it – to recognise terms of art, to follow the structure of an argument, to know when a large arc of text follows a single topic and where synonyms flock together usefully or otherwise.

One of the things that’s commonly imagined is that indexing is, in the age of Google, something that can be outsourced to a computer algorithm. Dead wrong. A concordance – essentially, an alphabetical list of all the words in a book with page references – can be done by a computer. But an index, to be useful, needs to be done by a human. In a book about the Middle East, say, an entry that said: “Syria 2, 3, 5, 6, 7, 10, 23, 25, 26, 27 … ” would be no use at all.

Humans who have understood the book are required to put in the subheadings that allow a reader to navigate the text – “Syria, interference in internal politics of Lebanon”, or similar. It takes a human to recognise the difference between “table” as a verb or “table” as a noun. It takes a human to think about what a reader needs from a book, to recognise a theme, to think about the difference between a passing reference and a key passage. Indexing is a work of interpretation: it is, however humble in the service of its object text, a work of intellectual scrutiny.

JG Ballard wrote a short story called ‘The Index’, which was the index to an imaginary book. Photograph: John Lawrence/Rex Features

Bad indexes are legion. Absent indexes almost more so. One of my correspondents recently bewailed the index of a major and bestselling recent book. “Entry for France – around 40 undifferentiated locators,” she complained. “Entry for Europe, over 90 page refs.” She concluded: “Looks like a concordance created by searching the PDF files.” I’ve been shocked by learning how often, now, publishers – especially American publishers – expect their authors to pay for an index themselves, do it themselves, or do without. Even a book as well-published as Michael Lewis’s recent The Undoing Project came indexless. I asked its UK publisher why. “They’d taken it from the US edition,” he said. That’s depressing.

Though I’m parti pris, obviously, I’d ask readers: if there’s no index in your book, or it’s no damn good, please do make a fuss. Let publishers know that they sell their readers and their authors short by skimping. And – parti pris even more, but to hell with it – I’d suggest that publishers use the Society of Indexers as a gold standard: it not only organises an annual conference but actively trains new indexers in the skills required, offers qualifications and a complaints procedure, and provides mutual support for those who do this important work.

Also, which is well worth remembering, a good index is often very funny. Those of us who take an interest in such things – and I think there should be more of us – end up hoarding favourites. Michel Foucault’s The History of Sexuality (volume two) contains the cherishable “Elephants, as example of conjugal virtue, 17”. A 1995 book on the computer contains a deliberately circular reference (normally an absolute no-no): “Loop, endless: see ‘endless loop’”; “Endless loop: see ‘loop, endless’.” JG Ballard wrote a short story called “The Index”, which was the index to an imaginary book; the late David Miller used it as the index to his anthology That Glimpse of Truth: The 100 Finest Short Stories Ever Written. And as my colleague in the society, Paula Clarke Bain, who blogs about comedy indexes, recognises, the indexes to Alan Partridge’s memoirs are at least as funny as the body text.

The entry for the subject of a biography is a potted biography in itself. I’ve always carried with me the memory of what may have been the first index I really noticed: that for Richard Ollard’s biography of the splenetic and self-important academic AL Rowse, which under the author’s name included “censorious and resentment”, “sense of rejection”, “solipsism”, “vindictiveness”, “egotism”, “fondness for money”, “liking for celebrities” and so on.

Debate goes on as to what was the first index. References to an index in English are found in Marlowe and Shakespeare. The Society’s scholarly journal, The Indexer, carried an article suggesting that the I-Ching, from China several centuries before the birth of Christ, may have been the original index. The profession has changed: our older members started out with handwritten index cards (yup: that’s why they’re called that) filed in shoeboxes, but most now use dedicated indexing software.

On National Indexing Day, don’t feel you have to make an index; in fact, I’d strongly advise you leave it to the professionals. It’s not as easy as it looks. But do, if you would, take a moment to appreciate one. You could even tweet your favourite under #indexday and consider it “recognition, act of”.