Janet and Allan Ahlberg are widely considered among the greatest of children’s authors: wildly successful, universally adored, and critically lauded (Janet was among a handful of children’s illustrators to have won the Kate Greenaway Medal twice). So I can hardly claim that their genius hasn’t been recognised.

However, there is a particular aspect to their work that stands out to me, and which has not, as far as I can tell, been discussed in the many articles written about them since Allan’s death last year. The delightful wordplay, the beautiful illustrations, the boundless creativity, have all been rightly praised, as has the inventive nature of their work, pushing the boundaries of the picture book medium with technical innovations such as the holes of Peepo!, or the adorably tiny envelopes and letters built into the The Jolly Postman.

But it’s another quality of their genius which has struck me over the many years that I’ve sat, reading and re-reading their books to highly sleep-resistant babies. And that’s the extraordinary level of effort and attention they put into creating a completely consistent and detailed world for every book.

In Peepo!, for example, every page is a literal window into a world so richly textured that even a character as minor as the neighbourhood dog manages to have a story arc of his own in just two appearances. Told through the observations of a baby, the book paints a picture not only of the domestic life of an ordinary family (father bringing in the coal, mother so exhausted by chores she needs an afternoon nap) but also of a country at war (barrage balloons over London, bombed out houses). Father, it is revealed later with unspoken poignancy, is only home on weekend leave and will be leaving again after the baby is asleep.

Each Peach Pear Plum is, on the face of it, a simple tale. In just 13 couplets, the characters from different nursery rhymes are linked together, in an early example of the literary mash-up, in a sequence of encounters revolving around the making and eating of a fruit pie. But aside from the book’s many other virtues (the interactive hide-and-seek game on each page, the powerfully soporific effect of the verse’s gentle, dispondaic rhythm), the really amazing thing is the trouble the Ahlbergs went to to create its setting.

Worldbuilding is the term used in science fiction and fantasy for the detailed mapping out of the setting and background of a story. You wouldn’t expect it in a book aimed at pre-linguistic children, containing just over 100 words. But the Ahlbergs did it. On Each Peach‘s title page, there’s an illustration of a landscape which you could easily flick past without paying it any attention, thinking it mere decoration. But a closer study reveals that it is a map of the entire territory over which you are about to journey. By comparing the pictures on subsequent pages back to that initial overview, you can see that every scene is a rendition of the same landscape from a different character’s point of view, accurate to the tinest detail of tree and shrub.

However, on the metric of the greatest narrative depth for the least ink expended, the most astounding achievement is The Baby’s Catalogue. Like the Ahlbergs’ other works, it’s deceptively simple, hiding deep sophistication and structure within the most humble of art forms. The Catalogue is a “word book”, essentially an illustrated dictionary, consisting entirely of paired words and pictures, to help with the very earliest stages of language acquisition.

That might sound like a medium so limited it doesn’t even qualify as literature. Yet, in the masterful hands of the Ahlbergs, The Baby’s Catalogue transcends its form to become a narrative vehicle, telling the parallel stories of five different families through a single day. Janet’s luminous and often hilarious illustrations achieve a depth of characterisation and detail of incident that many novelists fail to achieve with tens of thousands of words.

Perhaps I’m overanalysing. I admit I’ve spent a lot of time cradling a sleeping child, with nothing else to do but seek intellectual sustenance from the critical study of baby books. However, thanks to the brilliance and diligence of the Ahlbergs, that study was richly rewarded.