It's the one in which he finally worked out how to structure a very big novel.
It's not as (fitfully) brilliant as Bleak House. But neither is it such a mess. In fact it's not a mess at all.
The centre holds. Amy Dorrit is the most convincing of Dickens's angelic child women. He is beginning here to see more deeply into the psychology of self-sacrifice- to get a whiff of its pathology, and his questioning (which is also a self-questioning) gives her depth. She is a very needy person, very repressed. Her growing relationship with the equally needy and repressed Arthur Clennam is delicately painted and not without a muted and all but strangled-at-birth eroticism.
It is a novel of disillusion. Everyone is in manacles- real or "mind-forged" and the old Dickensian nostrums of kindliness and good cheer are very nearly not enough. Victories are allowed, but they are small private victories and the gloom of the prison house is never lifted. Only Great Expectations among Dickens's other novels is as honest and unremitting and adult in its vision. The hero and heroine are given a moment in the sun, and then at the close "go down" to "a modest life of happiness and usefulness". "Go down"- Dickens repeats the phrase to be make sure we catch on- is what condemned prisoners do.