The art historian and travel writer Michael Jacobs died last year, leaving a literary legacy at once irresistibly idiosyncratic and unobtrusively learned. In particular he wrote uniquely about the nation, culture and history of Spain, where he lived for much of his life and to which he was devoted. It is only proper that his final work should concern a supreme icon of Spanishness, Diego Velázquez’s masterpiece Las Meninas.

Everyone knows this painting. It must be one of the most famous works of art, reproduced all over the world, assessed in every reference book and truly representative of the Spanish tradition. For generations, scholars have analysed its brilliant technique and wondered about its meanings; Las Meninas is the supreme enigma picture, a riddle and a mystery, and for Jacobs it offered a lifelong challenge to his own complicated intellect.

The result is this fascinatingly confused and confusing book, which he never finished. Only a third of it is his. The rest is provided by his friend and colleague Ed Vulliamy in the form of an impressive introduction and coda, wrapped around the hundred-odd pages of Jacobs’s own work (and sometimes making it difficult to remember which is which).

Between the two of them, they tell us the origins of Jacobs’s obsession with Las Meninas and his purposes in writing the book: at once to explore the significances of this particular painting, which he considered one of the greatest of all pictures, and to philosophise about the study and enjoyment of art in general. The subtitle of the work is apt, because it really is like a long navigation through all the possible meanings, allusions, pleasures and puzzles of the painting.

Jacobs describes his fascination with the painting by way of other men’s responses to it, from Antonio Palomino in the 18th century, who called it ‘life itself’, to Charles de Tolnay in 1949, who suggested it was an allegory of artistic creation, and to the American Jonathan Brown in 1978, who thought it was merely part of a campaign for Velázquez’s ennoblement. Renoir said it made him want to give up painting; Picasso painted fifty-seven variants of it. Michel Foucault seems to have been a major influence on Jacobs’s thinking about the picture; David Davies of the Courtauld Institute, Jacobs’s alma mater, thought it an ‘evocation of a world in which everything is about to happen’.

Hence this book’s title. Here perhaps I should remind you, at the risk of impertinence, of what the painting actually depicts. It shows, in richly opaque colouring, a room in the royal palace of Philip IV of Spain, Velázquez’s patron and friend, in which the artist himself is at work with palette and gigantic canvas, pausing for a moment, brush in hand, to look directly at the viewer. In the centre of the room is the king’s daughter, Infanta Margarita, attended by ladies-in-waiting (las meninas) and by various familiars of the court, including two dwarfs and a handsome mastiff. Reflected in a mirror, one can mistily see two people whose faces are unclear but who seem to be the king and queen themselves. Prominently in the background a man is sharply delineated half in, half out of a brightly lit doorway. At first sight, you may think it no more than a brilliant conversation piece, but the more you consider it, the more its mysteries emerge

What’s it all about? Most of the people in the picture seem to be looking at us – the viewers – and the monarchs in the mirror would appear to be the subject of Velázquez’s work-in-hand. Nothing quite fits, though. The perspectives are odd, with multiple vanishing points. If the king and queen are sitting for their portrait, they should be where we ourselves are standing, but they would not then appear in the mirror. Sundry historical and personal allusions further complicate matters and the presiding strangeness is the spell the picture exerts upon almost everyone who sees it – an effect partly of its beauty, partly of its puzzling nature.

True to his own life’s highly original pattern, Jacobs’s personal contribution to Everything Is Happening takes the form of a kind of travel book. It concerns his continued search for the truths of Las Meninas by way of journeys, meetings, conversations and conjectures, and much of it tells the story of the picture’s physical history, from its completion in 1656 to Jacobs’s own last sight of it shortly before his death.

The canvas itself (which is very large) has not had an easy ride. For years it was considered of lesser importance than Velázquez’s The Surrender of Breda, and it was only in the late 19th century that it acquired its almost sacred place in the Spanish national consciousness. Once it was almost burned, often it was disregarded, for a long period it was propped against a wall somewhere, and even after it had acquired its own shrine-like quarters in the Prado Museum in Madrid, it spent three years in Swiss exile during the Spanish Civil War in the 1930s.

Jacobs’s narrative of his travels in search of the picture’s meanings is a delight. He meets everyone who might have a view to express about the old enigma, including the man who escorted the picture on its train journeys to and from Geneva. His gift of cheerful observation, combined with profound intellectual interpretation, is as compelling in this brief valediction as ever it was in his creative prime. Helped along as it is by Vulliamy’s affectionate (and extremely learned) commentary, the book gradually resolves itself into the quest the subtitle suggests.

What with the contributions of so many thinkers, scholars and artists, by the time I neared the end of the book I was beginning to think it would leave me none the wiser about Jacobs’s own interpretation of the painting. But at the very end, we learn that he believed the ultimate focal point of Las Meninas – and so perhaps its ultimate message – might be that figure in the background, partway out of the open door, looking thoughtfully back at the goings-on behind – and possibly looking at us too. Is it Diego himself, leaving this life with all its pretensions and confusions, its pomps and its ambiguities, for another, clearer world outside?

Velázquez died four years after completing the picture; if Michael Jacobs had lived another couple of years, his friend tells us, he might have completed this endearing book.