There are two ways of looking at this: 1967, when the book was first published, and the 2020s.
In the late 1960s/early 70s Smith was greatly admired by many of the literati who occasionally turned to look at children's fiction. Yes, they pointed out that it was derivative. It has Charles Dickens stamped all over it despite its being set in Georgian London. Set against that is Garfield's use of English. There are few holds barred when it comes to tussling with a young reader's literacy. These may have been the days before the internet, with instant answers available to the curious, Garfield simply assumes that any young reader with more than half a brain would ask someone knowledgeable, a schoolteacher would be ideal in those days, to explain a word or elaborate passage. If the teacher was anything like my revered Mr Caldicot he would talk me through it and then demand an essay on the subject to prove I had been listening.
To choose a passage at random. This is at the beginning of the story when Smith has made his way home to the cellar of the Red Lion Tavern, having witnessed a murder and stolen the document that will change his life:
“'Hullo, Smith! Not nubbed yet?'
Deep in thought, he ignored the landlord's pleasantry and made for the cellar steps. From below the tallows gleamed yellowly and cast strange shadows on the wall. He began to descend, when -
'Stand and deliver!'
A voice like twelve o'clock of St Paul's roared from the heart of the cellar! Smith started, missed a step, and came down the rest any way but on his feet!
A dangerous, glittering, murdering adventurer of a gentleman in green stood before him, aiming a pistol the scope of a cannon directly at his head! It was his friend, Lord Tom, the high toby, come on a sociable visit.”
This is Garfield saying, 'This is the way I write. Stick with it or go back to Enid Blyton.' I stuck with it and reaped a reward of many more immensely enjoyable books.
The plot is a decoy. Garfield loved stories of lost inheritance but here it is a long time before Smith realises he won his own lottery while undergoing pursuit by two underworld thugs, imprisonment in Newgate, the loss of friends along the way and threats to his own life. Some episodes are a little theatrical, Smith's escape from gaol is quite an achievement for an undernourished twelve-year-old and Lord Tom is straight out of pantomime, but they fall into a swinging flow of the tale and it is a tale that carries the reader along through adventure, humour and pathos. It was only runner-up for the Carnegie Medal having the misfortune to come up again Alan Garner's The Owl Service. It's debatable which was the better.
In the 2020s styles have changed. Garfield's language is far too ornate and literary, simply from an earlier time. The cast of characters is strictly white. Despite being set in the eighteenth-century there is not even a black servant in one of wealthier households. Slavery and plantation agriculture were dominant forms of income for the well-heeled, yet it and Black people may as well not have existed in Smith's world. Sexuality, where it exists at all, is strictly conventional. There is not even a gay comedy character. In fact, the whole of the story is conventionally 1960s in terms of inclusion. There are Londoners divided into rich and poor, honest and dishonest, and that has to do.
The story is firmly a boy's adventure though there are some strongly drawn female characters. Smith's older sisters, Fanny and Bridget stand out. They could have been presented as prostitutes with hearts of gold, but they are not. Fanny and Bridget are honest, hard-working young women who earn a short living repairing and altering the clothes of recently hanged villains and then reselling them. They are two of very few people who really care for Smith and his well being. Mr Mansfield's daughter is a cleverly drawn character. On the face of it a typically Dickensian angel of mercy caring for her blind father while ready to emotionally exploit the situation knowing that all her father is aware of is the sound of her voice. Her expressions tell the rest of the world how she grudges her lot in life. As women they work well to bolster the story but only in the background and often providing light relief rather than anything of significance.
The moral of the story that theft can pay in the end if your lucky is certainly still true in 2022, however, I think we like to feel that such concepts belong in the past and should not be advertised. For a 2022 reader the story would probably be seen as too old-fashioned for its own good and is best left to gather dust. Well, the 2022 reader is missing out.