Agnes Grey is a good but not a great book. I found it to be inferior to Anne Bronte's other major work The Tenant of Wildfeld Hall and Anne is certainly not a genius unlike her more famous sisters. The book concerns the life of a young governess, who is disillusioned by her experiences in that role. It suffers from a kind of whininess. While the life of a Governess in the 1840s was not all that great it was certainly better than the life that most people led. There is a constant sense by the heroine (it is written in the 1st person) that she is being treated below her worth, but that is coupled with a lack of awareness of class issues in others. Coupled with that is a kind of priggish Protestantism that reminds me of Jane Austen at her worst.
Having said all that Anne was raised in an atmosphere of genius and a lot of it has rubbed off. Her prose is good and often excellent. The book moves well and is structured well and the character portraits good (although one often senses that they are unfair). I believe that this book would not have lasted if it wasn't for her more famous siblings, but having said that it is still a good book and an interesting look at the upper classes.