This book is a celebration of trees and bigger trees and some of the biggest landscape paintings in art history. It is about much more than that, but trees are at its massive, strongly beating, very English heart, and David Hockney's discovery of them is an invitation to us all to look better, see better, enjoy more.

The beautifully illustrated (and very fairly priced) volume takes the form of conversations with Hockney's art historian friend Martin Gayford (they are designated on the page as DH and MG). MG prompts DH to talk about his move from California to Bridlington, his preparation for his forthcoming exhibition at the Royal Academy, his views on the differences between painting and photography, and his ongoing love affair with new digital techniques.

Hockney loves trees, he loves gadgets, and he loves to paint. The combination of these enthusiasms is producing, in his 70s, some huge works on an epic scale of mind-changing colour and glory, as well as some miniatures drawn on his iPad. These domestic sketches  the view from his bedroom window with a street light, his bedroom curtains, a bowl of flowers, a cactus, an ashtray  appear as if by magic nearly every morning in the inboxes of his friends. This man is blessed with great gifts, and he shares them with great generosity. He says he has found a new lease of life. "I would never have expected to be painting with such ambitions at this age. I seem to have more energy that I did a decade ago, when I was 60." His work rejuvenates him, it rejuvenates us all. DH is very inclusive.

Trees are long-lived, they become old friends and then they outlive us. DH claims "they are the largest manifestation of the life-force we see. No two trees are the same, like us." MG includes in his commentary Constable's description of the "young lady" ash tree on Hampstead Heath, together with a reproduction of Constable's 1821 Study of the Trunk of an Elm Tree, with its extraordinary details of bark, and he also quotes from Colin Tudge's The Secret Life of Trees, a book both he and Hockney admire. Trees are "like human figures in the landscape, vegetable giants, some elegant, some heroic, some sinister ... but they are also remarkable feats of natural engineering, capable of holding up a tonne of leaves in summer against the forces of gravity and wind". This observation draws Hockney on to speak of the spatial thrill of trees and their capturing of light  a winter tree helps you to sense space, a summer tree in leaf is a container of light  and also to the theme of the changing of the seasons and the changing light of every day.

Hockney has learned to watch the seasons acutely since he moved back to his native Yorkshire. He knows when to catch the hawthorn in bloom, and gets up early with his nine-camera team to film leaves turning colour in the autumn and bare trees decked with snow. He films and paints the same deeply familiar tunnel of trees and bushes and notes how the position of the sun changes through the year  a natural phenomenon he'd never noticed in California. (Maybe it doesn't happen in California: Bridlington is, as he often points out, quite a long way north.)

MG, on a 2010 outing to Glyndebourne with DH to see a revival of the 1975 production of The Rake's Progress with Hockney's original sets, remarks as they sit in the grand and formal Sussex gardens on a perfect summer day that the landscape is a "huge natural theatre that is being lit by the sun and the weather in an infinity of varying ways". DH assents, but is soon drawn back to the subject of his humble tunnel on a misty morning: "You get a marvellous range of greens, more detail in the cow parsley. If it had been a sunny day, it would have been a little flatter ... a morning like that is a great rarity." More detail in the cow parsley: that's so English, that's so good.

The landscape of the Yorkshire Wolds is modest, unspectacular, unfrequented, and despite his long absence Hockney says he is now learning to know it as thoroughly as Constable knew East Bergholt and Dedham  he has gone back to his roots. But he hasn't gone to Earth. He is remains deeply interested in the work of his predecessors, and full of lateral thoughts about them. His response to one of the fathers and masters of the outdoor landscape, Claude Lorrain, is fascinatingly quirky: he is full of respect for Claude's trees and the delicacy of his foliage ("It probably isn't that natural, but it looks it") but at the same time he is determined to apply revolutionary photographic Photoshop techniques to "restore" and recreate one of Claude's larger and lesser known paintings, The Sermon on the Mount from the Frick Gallery. Although I have had the good fortune of a an early view of his vast and colourful version, then in his huge rented warehouse on a Bridlington industrial estate, and entitled (like this volume) A Bigger Message, I was too over-excited and over-awed to take in what was happening there. I understood in my ancestral bones the Yorkshire trees and the shady tunnel, but this strange vision in vivid reds and green and blues was like nothing I had ever seen before. "It's not oil paint," as he explains to Gayford, but what is it? It is a virtual Claude, revealing, as the Frick version did not, "the lame and the blind in a pit".

The weird combination of ancient landscape and new gadgetry is exhilarating. Hockney will try anything. He speaks with the greatest admiration of Van Gogh's human vision, his fine draughtsmanship, the speed and energy of his brush strokes, his northerner's joy in the clarity and light of the south (similar to Hockney's own youthful delight in California), his ability to transform the dullest subject, his love of the nondescript, his letters with their little sketches like drawings of drawings  "Van Gogh could draw anything and make it enthralling ... a rundown bathroom or a frayed carpet." Van Gogh distrusted photography, would never pose for a photographer, but his fellow artist DH claims confidently in one of his frequent texts to MG, he would have gone for the iPad. "Van Gogh would have loved it. He could have written his letters on it as well ... Picasso would have gone mad with this."

And so Hockney goes on sketching in his old-fashioned, comfy, hi-tech seaside home. He draws the washing-up in the sink, his own bare foot with its slipper by its side, his cloth cap, just as they happen to catch his eye. And in the warehouse on the estate, the bigger trees and the bigger message grow and grow.

Margaret Drabble's The Millstone is published by Penguin.