There has been a lot of Gainsborough about recently, notably the three exhibitions that ran concurrently in Bath, in Sudbury and at the National Portrait Gallery. If these have been the tip, Hugh Belsey’s superlative catalogue of Gainsborough’s portraits is the whole of the iceberg.

Printmakers excepted, Gainsborough was, as Belsey amply demonstrates, the most prolific high-end commercial artist of the 18th century, with Reynolds his only rival. For commercial he was: his portraits were painted for a market, and the only way he had to regulate demand when it surged (apart from being rude to sitters) was to put up his prices; conversely, when business was slack he painted his family and friends. Rising prices only stimulated patrons in that affluent age, so the trick never worked for long. Gainsborough’s gritty, complicated existence, which included high expenditure of time hors concours with musicians and thespians, sharpened his speed of execution, but led him also into delays in completion and delivery.

The 18th-century portrait market required clarity, consistency and clear likeness. Gainsborough’s patrons expected that the commissioned portrait be comparable to those earlier examples that had led them to choose him in the first place. Gainsborough needed his sitters to be present and engaged – his dissatisfaction with his attempts at painting Shakespeare is evidence of that. Sitters such as the convivial William St Quintin and the gruff Uvedale Tomkyns Price amused and roused him, while others such as the impatient George Lucy and the distracted William Pitt presented difficulties. ‘Now Damn gentlemen, there is not such a set of enemies to a real artist in the world,’ he wrote.

As he moved from one side of the country to the other, from Ipswich to Bath in 1759, and then in 1774 from Bath to London, the stylistic changes that followed brought him one success after another. Each move produced a refreshment of manner, an increase in scale and an even grander class of patron: little portraits of country clerics in Suffolk in the early 1750s, full-lengths of royalty in London thirty years later.

Belsey’s catalogue is a terrific achievement. He has been building it, as he notes in the acknowledgements, since the early 1980s, and this sumptuous two-volume boxed set is a landmark by any measure. While at £150 it won’t be a bestseller, it will find a permanent home in libraries across the world and have a lasting influence on the study of Gainsborough. I would remind lay readers tempted to buy it that it is only the price of two or three tanks of petrol for the average family car.

Painting his family, as the recent National Portrait Gallery exhibition charmingly revealed, was one way Gainsborough had of relaxing, absenting himself awhile from ‘the continual hurry of one fool upon the back of another’ and from those ‘damn gentlemen’ who both plagued him and provided him with a splendid income. It also, as Belsey shows, helped him loosen his manner and relax his grip, allowing him to invent compositional modes that are illuminating and have the intention to engage and flatter. Two double portraits of his daughters, one with a butterfly, one with a cat, are masterpieces of love, characterisation and concern. His family portraits emboldened him to draw livelier poses from his sitters and showed him how free-running brushstrokes could give his paintings the breath of life. Extreme time pressure on him was, as Belsey points out, a contributory factor in the development of his style. He had to paint quickly to earn his money; the vivacity and sparkle that came from his brush gave later generations of artists the courage to paint with similar freedom. Belsey puts it concisely: ‘At his best Gainsborough’s work seems effortless, with form perfectly realised, and his command of his materials and the variety of brushstrokes … produced any effect he wished to achieve.’

Listed and reproduced here are 1,031 portraits of men, woman and children, ten portraits of dogs and another twenty-five or so ‘fancy’ pictures (country people within landscape) and copies of works by Gainsborough’s beloved old masters, principally Rembrandt, Rubens and van Dyck. Gainsborough’s 180-odd landscape paintings (catalogued in 1982 by the late John Hayes) and countless drawings are beyond Belsey’s remit, so these volumes are only one of the tools we require to bring us eye-to-eye with this extraordinary artist. Belsey points out that Hayes’s landscape catalogue created an artificial division in our understanding of Gainsborough’s output. This only goes to demonstrate the impossibility of trying to bottle such a fecund genius. The extreme contrast in the numbers of portraits and landscapes makes it clear that, as much as Gainsborough might have talked about longing to get out into the country to paint landscapes, his contemporary fame and his living depended wholly on his portraits. It was oddballs such as St Quintin who encouraged him to paint landscapes.

These volumes allow the reader to travel the complex system of pathways through Gainsborough’s art and be led by cross-references from sitter to sitter. Mysteries in Gainsborough are solved, such as the identity of the subject of The Blue Boy: not so much a whodunnit as a whowasit. Each catalogue entry begins with a detailed description of the sitter, costume, pose and setting, and represents a masterclass in fashion classification. Here, for example, is Belsey’s account of Mary, Countess Howe’s costume:

Some polite arguments go on in the text between Belsey and his fellow art historians. For example, he discounts as ‘coincidental’ the similarities, noted by Martin Postle, between Gainsborough’s portrait of his daughter Margaret as a gleaner (in the Ashmolean Museum) and a painting by Ribera in Naples. There is even an unwarranted expression of shock in the footnotes to his discussion of Gainsborough’s celebrated portrait of Mr and Mrs Andrews at my own observation that the painting contains distinct sexual symbolism. ‘Pure sensationalism,’ he blusters. A catalogue such as this is the natural home for the airing of differing views of this kind.

We can trace within the pages of this work the changes to the canvases, both those made by the artist himself – revealed by the ‘pentimenti’ that show up the second thoughts brought on by the rapidity of his initial actions – and those imposed by later owners. The group portrait of the three eldest daughters of George III, Charlotte, Augusta and Elizabeth, for instance, was cut down to fit over a door. A similar mutilation was visited on the formerly full-length portrait of the Duchess of Devonshire. This is now, at last, at Chatsworth, but not before an eventful 19th-century life. Before 1841 the portrait was owned by a schoolmistress, who cut it down so it would hang over her fireplace. It was stolen from a later owner, trimmed again at the bottom and sides, and folded up and touted around America hidden in the false compartment of a trunk. The brave boys of Pinkerton’s Detective Agency caught up with the thief and in due time the heavily knocked-about duchess came home.

Biographies and exhibition catalogues can only go so far. A book such as this is therefore vital in providing plenty of elbow room for discussion and illustration of a great artist’s work while also allowing early 21st-century interpretations to be set out for generations to come. Belsey’s (and my) generation of art historians have benefited profoundly from the work of earlier monographists, and while research fashions ebb and flow there is always a need for life’s works such as this.

Let us give Gainsborough himself the last word. In a letter to his musician friend William Jackson, he wrote: ‘One part of a Picture ought to be like the first part of a Tune; that you can guess what follows, and that makes the second part of the Tune.’ Gainsborough is describing the effect that, a century later, the Impressionists exploited: what the artist expresses, the eye completes.