Allan Ahlberg is considering some illustrations for his next picture book, Mockingbird. “Look at that little scowl, and there’s that serious baby look, that’s really nice, that’s classic.” From a distant house with a bird flying by, the reader moves into a delightful domestic scene inside the house, the fluid arabesque of Paul Howard’s beautifully simple illustrations enhancing the movement and rhythm in the language.

As Ahlberg reads the text, he moves his hand across the drawings, exaggerating the turn of the page, trying to show how crucial is the design of the whole book in giving life to the language. Mockingbird, set in America in 1910, is an inspired reworking of the lullaby, yet another example of how he can adapt archetypal nursery songs to stunning effect to preserve the moments of babyhood cherished by adults:

She’ll wipe your face and dry your hair,

Sit you up in your own high chair,

Tie your bib and - for goodness’ sake -

Daddy’s gone and baked you . . . a birthday cake.

Hush little baby, don’t say a word, Mommy’s gonna buy you . . . a Mockingbird.

Words alone will not do for Allan Ahlberg. He is obsessively concerned that the pictures should extend the narrative and lift the story. It is an obsession born from his partnership with Janet Ahlberg, his late wife and illustrator. Since her death two years ago from cancer, he has struggled to find a new way forward.

Breaking his old alliances with Penguin and Reed, he turned to Walker, which will publish Mockingbird next year, searching through its ranks of fine illustrators to find a new working partner. He said: “I couldn’t bring new work to the editors that Janet and I had worked with. It just felt too bad, at least to begin with. I will do work with Penguin and Reed but I had to make a fresh start . . . I have to find other people to collaborate with.”

Janet and Allan Ahlberg wrought wonderful magic together. Over 20 years they published more than 100 children’s books, many of them ranking among the great children’s classics of this century. Peepo!, Each Peach Pear Plum, Burglar Bill, Cops and Robbers and The Jolly Postman have enriched the language and imagery of childhood literature with their gentle rhyme, wit and profound humanity.

Few parents can look at a Janet Ahlberg bedroom scene, for instance, with clothes piled high on chairs and the end of the bed and baby things scattered around the floor, without the enjoyment which comes from the shared experience of those chaotic, early days of parenting. Children, too, love to explore the wealth of miniature detail in her pictures.

Professor Margaret Meek, emeritus reader in education at the University of London Institute of Education, has commented on their “small, infinitely revealing vistas of English social life and ‘innerly’ ideologies . . . You have to look and read carefully, as children do, so that the words expand the images and the images ‘thicken’ the meaning of the words. No explanation conveys this accurately; only the books do.”

They had planned many more books - more Jolly Postman titles and a book based on comic strips in which one strip leaked into another and the stories got mixed up. “Janet didn’t want to leave,” Allan Ahlberg writes in Janet’s Last Book, which he produced in her memory for friends and family. “The party was still going on. There was a manuscript waiting, proofs in the pipeline . . .” Janet’s Last Book is full of her artwork and roughs, the best from the thousands that he and Jessica, their daughter, found in Janet’s studio.

As a writer and an illustrator, Allan and Janet were a perfect match. Starting again, he says, is like entering another life.

“It’s like I was riding a tandem for 20 years and now I’ve got to ride a bike on my own . . . After Janet died I didn’t do anything much except drink and sit around for about a year and then, last January, I tried to buck myself up.”

Mockingbird, he admits, would have been “right up Jan’s street”, but as time goes by he becomes more cheerful and optimistic about doing the book without her, and his face lights up when he looks at Paul Howard’s illustrations. The relationship will be different, but he hopes it can be fruitful. “What I’m trying to discover in the next few years is: ‘Can I invent a process with other people that makes good books?’ That’s one thing and I think I can do that, and the second thing is: ‘Can I enjoy it?’ I don’t think I can enjoy it the way I did with Janet. Paul Howard, for example, is a really nice bloke . . . I think we can get on well and become friends, but in the end, in the finish, it won’t feel like our book - well, I’ve just got to do what I can with that.

“I get waves of dejection, of course, but I have no doubts about the advantage to me of trying to work if I can and I can’t simply write a story and give it to somebody else and forget about it.”

The Mysteries of Zigomar, a collection of poems and short stories, will also be published by Walker this year, illustrated by John Lawrence. It carries all the Ahlberg hallmarks: wry observation, fanciful and hilarious variations of traditional tales, school poems, funny post-modern tales which call the writer to account - the spark is still there. The books that Allan Ahlberg produced without Janet, usually for older children - Please Mrs Butler, Heard it in the Playground - have also been bestsellers, applauded by all who are concerned with children and words for their wit and compelling language, encouraging children to be poets, performers and writers. But, he says, even the solo books were the offspring of that partnership.

Allan Ahlberg believes he was programmed to be a writer, despite being brought up in a house with no books (he was the adopted child of a poor working-class family in the Black Country). He was desperate to write radio plays or novels but, until his early thirties, “could never get past the first page”. After trying a weird and wonderful string of jobs - gravedigger, soldier, plumber’s mate - he trained to be a teacher (meeting Janet at teacher-training college) and taught for 10 years, including a period as the head of a two-teacher country primary. It was only when Janet, then a jobbing illustrator for non-fiction books, asked him to write a story she could illustrate, that the door opened. “It was as though I was one of those little wind-up clockwork men and she turned a key in my back.”

When Janet was busy with their picture books, Allan worked with Fritz Wegner, Colin McNaughton and Andre Amstutz, among others, but she was still his first critic. He said: “She was the first fork in the road, whether the book got done or not. I’d write a script and I’d show it to Jan and the process was that I’d feed off her response. She had a bad time with me because I didn’t take criticism well. However she was a gentle and kind woman and put up with me. She had an instinct for whether something was good or not and if she liked it that usually meant I liked it. The books that we both enthused about were the books that got done.”

In their elegant Victorian house in a village outside Leicester, Janet worked over the garage late into the night, Allan in the garden shed early in the morning. “I’d come at her with some writing and she’d say ‘Yes’ and then she’d do some little drawings and I’d look at her drawings and the book would kind of grow for months and months here, just between the two of us, and then we’d make a tiny mock-up of how the whole book would look, for the publisher.”

The Ahlbergs sought control of the book production process. Allan still does: he is involved in pictures, endpapers, covers and blurbs. Jessica, who has inherited her mother’s ability, has stepped in to make the dummy for Mockingbird with her simple, enlivened line drawings.

So how will his work develop? “I don’t really know. I think I’ll carry on with the range of things I have hitherto done - poems, stories, long and short. I’m still trying to find clever ways of playing with the form of the book - ways of opening it up, reinventing it.”

Allan Ahlberg has cleared his bedroom, he’s stopped spraying Janet’s perfume around, he’s got rid of the bed and created a bedsit. “Sometimes I just pile out of bed and sit down straight away and work.

“The way I look at it is this: for 20 to 25 years I made a shelf of books with Jan, and over time I will make another shelf of books. That’s about as far as I can see.”