|The Bronte Sisters|
by Anne Brontë
Published in 1847
It is impossible to write about Agnes Grey without also considering Jane Eyre and Wuthering Heights. Agnes Grey and Wuthering Heights were actually published together as a "triple decker" with Wuthering Heights comprising the first two volumes and Agnes Grey the third volume. The three books were written by the Brontë sisters: Charlotte (Jane Eyre) and Emily (Wuthering Heights.) All three were published in 1847, and while Jane Eyre was the "hit" out of the three, an Ngram viewer looking at all three titles at once shows that Wuthering Heights trailed in popularity from the time of publication to the 1940s before pulling even and trading places with Jane Eyre as "most popular novel published by a Bronte sister in 1847" into the 1980s. Of course, the last 30 or so years have been all Jane Eyre- which you can see in an Ngram comparing the two titles, with Wuthering Heights holding down a respectable second place.
On the other hand, Agnes Grey is a distant, distant third place although Agnes Grey has a pulse, it is not a strong pulse, and I would argue that it is, at best, a "minor classic" if only by virtue of the relative lack of popularity to the two other novels published by her sisters in the same year as Agnes Grey.
It is simple enough for the reader to see the influence on Anne Brontë by William Makepeace Thackeray's Vanity Fair- it's an influence that is also strong in Jane Eyre, but less so in Wuthering Heights, which is more stylistically in tune with early Gothic/Romance fiction.
Agnes Grey is about the experiences of the titular character, who serves with a couple different families as a governess. Agnes Grey is the most auto biographical of the main characters of the Brontë sisters out of the three titles published in 1847. Anne Brontë actually did work as a governess for five years. As the wikipedia points out, Agnes Grey also contains elements of the more stylistically advanced "Bildungsroman"- a coming of age story where the main character grows as a person and learns important life lessons during the course of his/her adventures.
A half century after the publication of Goethe's, The Apprenticeship of William Meister (1796) and Maria Edgeworth's Ormond, the bildungsroman was not fully established in the world. At the same time, the picaresque- the shapeless "life and adventures" style of novel that ruled the 18th century- had fallen into sharp decline.
If you look at the "classics" that were published between the turn of the 19th century and the 1840s, there is alot of Sir Walter Scott and his followers (James Fenimore Cooper, Alexandre Dumas, Stendahl, Hugo, arguably Balzac) the emergence of Charles Dickens as a force, Edgar Allan Poe and is about it. It's important to recognize how fresh and unusual Agnes Grey must have been to the initial Audience for the work- it's certainly more subtle then either Jane Eyre or Wuthering Heights and were it written by someone other then the sister of Charlotte and Emily Brontë it would probably get more contemporary attention, but alas.