This is a semi-autobiographical account of Dostoyevsky's time in prison. It lacks a sense of time or a plot. The first part of the book has a sort of temporal structure, but Dostoyevsky leaps out of it so frequently, that it is completely undermined. The narrator is strangely distant, he speaks mostly of the other prisoners and writes in a disassociated manner. The horror of the situations is removed and instead there is a dispassionate wondering confused failed analysis instead.
All of that is appropriate and probably inevitable, because it captures the timeless horror of prison from which disassociation is the only escape. Because it is partly autobiographical the disassociation was almost certainly felt by Dostoyevsky while writing it.
However, while it is as authentic a book about prison as I have read, it does not always work as well as it might. Dostoyevsky does not have access to some of the writing techniques that had been developed to convey timelessness, incomprehensibility, horror and disassociation, so he is working with the techniques that he had. While those techniques were reinventing the 19th century novel, they are not quite up to the task here.
I once read that every novel that Dostoyevsky ever wrote could have been called "Crime and Punishment". That is certainly true here. The books primary interest is probably to those interested in his work, to show the source of the trauma evident in his other writings and at least part of the reason why Crime and its subsequent punishment dominate his work so thoroughly.
The result is a book that is not amongst his very best, but which is of interest to anyone who likes Dostoyevsky. I would recommend reading all of Crime and Punishment, The Idiot and Notes from the Underground first (I think that the Brothers Karamazov is overrated, although probably better than this). A good book, but one which does not quite succeed, through no fault of the author.