It was an idle click on an unpromising website that first directed Nora Crook towards the most exciting and unexpected discovery of her distinguished academic career. Crook, a professor emerita at Anglia Ruskin University and expert on the Romantic period, was researching an obscure 19th-century novelist when her internet search brought up a listing for 13 documents at Essex Record Office, catalogued under the tantalising words: "Letter from Mary Wollstonecraft Shelley".

"I thought: 'What is this?' and clicked on the link," she said. "I knew right away they had never been published before."

Thanks to "pure serendipity", Crook had chanced upon the largest collection of unpublished letters by the author of Frankenstein to be discovered in decades.

The letters date between 1831, nine years after the death of her poet husband Percy Bysshe Shelley, and 1849, when Mary Shelley was already unwell with the brain tumour that would kill her two years later, and show a woman who was skilled in charming favours from friends, bursting with pride in and concern for her teenage son – and not unconcerned with frivolities. A last-minute ticket to the coronation of William IV in 1831 necessitated a 3am visit from her hairdresser; she attended the event sporting a plumed headdress ("The whole thing was wondrously splendid – Diamonds & cloth of gold grew common to the eye.")

The later letters, written in an increasingly scrawled hand, are short and distracted, full of apologies for her failing memory and powers. To a number of them is still attached a blob of scarlet wax stamped with the author's own seal – one that was not previously known, according to Crook.

The "nondescript-looking" missives were written to Horace Smith and his daughter Eliza. Smith was a stockbroker and wit who had been close to the poet Shelley and whose family befriended his widow after his death, and the discovery of the letters is a particular surprise, said Crook, because he had a habit of destroying correspondence.

"The Smith connection has been known but this little bit of the jigsaw hasn't been," said Crook. "A few things that had been inferred by scholars can now be confirmed. But what is nice is that Mary Shelley's personality emerges. We see her as very loyal to the Smith family, very grateful and very attentive to Eliza – I don't think that friendship has ever been fully documented." There is also, notes the professor, Shelley's "charming wheedling side", as she cajoles Smith into favours.

In one letter she asks her friend if he will edit a manuscript of an author, Edward Trelawny, which has been deemed too racy by his publishers (Trelawny has agreed to censorship so long as a woman doesn't do it, she notes). In another, she asks him to let her publish letters in which her husband had expressed hostile views about religion, and which Smith had previously refused to release. In both cases, he agrees.

Perhaps most touching is her pride in her son, also called Percy and the only one of her children with Shelley to survive infancy. "Percy is growing up a very fine young man & developing tastes & talents that would remind you of his father – though he has not that touch that at once made Shelley angelic & unfortunate." After her son goes to Cambridge, she writes, "he is getting all that we could wish – he is getting very liberal – & has so much character & talent – though still shy – that I have every hope for his future happiness".

His sweet nature "repays me for how many years of sadness", she writes later, though she also admits: "I am mortified he is not taller."

Her own worsening health hampers the communication in later years. "Today I have been down stairs & taken an airing for the first time – I hope I shall have no relapse," she writes to Eliza in 1846. "This note looks blotty and invalidy – indeed my drive has tired me."

Perhaps surprisingly, however, notes Crook, "she doesn't mention Frankenstein at all, even though some of the letters are from 1831, the year in which Frankenstein was republished."

The letters will be published shortly in the US-based Keats-Shelley Journal. For the woman who unearthed them, their discovery is "very satisfying. Maybe nothing like that will ever come my way again, so I am enjoying this. I just wish that every researcher could have a lucky find like that too."