Here's the paper sleeve on the book cover:
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"This is an untold story about the siblings. After having obtained his revenge, stuck in a black dream, in the depths of his eyes is his brother's figure from the old days... what did the little brother see in the village of the 'Howling Wolf', that he reached by relying on something left to him by his dead brother? And then, the beginning of his revenge."
Here's the first page of the book:
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Here's the paper cover of the book, once you remove the outside cover:
And here's the inside of the cover (they're all Itachi and Sasuke images from the manga):
A brief summary (though without many spoilers):
The novel is set after Itachi and Sasuke's fight, and before the formation of Team Hawk.
By relying on something left to him by his dead brother, Sasuke visites the village of the 'Howling Wolf' - where he gets involved in some events and is convinced once again of his brother's feelings towards him; his decision to take revenge hardens.
The original characters of the book are a pair of chemist siblings, from whom Sasuke buys his brother's eye drops and medicines.
Seeing the big brother protect the younger brother, Sasuke is once again reminded of Itachi and is very pained; the two siblings reflect his own feelings, and he instinctively saves them and they sort of become friends - even eating some onigiri together in a tea house.
Apparently, in this novel Sasuke gets many 'cute' moments where he acts much more like his first part self than his second part self... but the atmosphere of the book is overall pretty dark.
Here are some Itachi/Sasuke tidbits from the novel that Mutsumix made sure to bookmark:
Thinking about Itachi, Sasuke gives himself a poke on the forehead while crying.
Sasuke had apparently tried to ask Tobi where he buried his brother's corpse, but in the end he wasn't able to do it.
Instead of paying attention to the meaning of the kanji written on Itachi's medicine, Sasuke is fascinated by the fact that his brother was the one who wrote them.
Sasuke stuffs himself with his brother's favourite dish (konbu onigiri).
Sasuke asks the chemist siblings to please tell him about Itachi.
At hearing of how much Itachi was thinking about him, Sasuke starts crying. When someone remarks on his tears, he tries to brush the whole thing off.
When he thinks once again about the fact that to get the Mangekyou you have to witness the death of a loved one, Sasuke is surprised to realize that his brother knew exactly how much he loved him.
Sasuke remembers the night that Itachi cried.
After awakening the Mangekyou, Sasuke feels Itachi near him like never before. "Even if the whole world were to turn into my enemy, I'm not alone. Nii-san... I've received what you left me..." Mutsumix jokes that getting one's eyes transplanted must turn you into a yandere...
After recognizing the boy in his dreams as Itachi, Sasuke can't stop his tears. "Even if you were to sever every single one of your bonds... even so, from here on after, I'll always..."
"For me, Itachi... is the only brother I've ever had."
As you can expect, Mutsumix liked the novel very much.

Not only because it touches a lot on Itachi and Sasuke's brotherly bond, but also because the villain was a real villain (her words) and because Sasuke shed tears and showed a lot more of his human side.
Oh, and here are some more 'general' tidbits I got from 2ch - since I still haven't been able to find a complete summary of the entire thing:
Sasuke calls the little brother 'usuratonkachi'.
He can't read some kanji (not sure if this is in general, or because he's starting to lose his sight).
When he gets into the village, he gets told that his complexion is bad.
From afar, a little girl comments that he looks anguished - but her mother warns her not to look at him.
He doesn't react at other people's jokes.
He invites the 'little brother' character to the tea house and (together?) they eat some unappetizing dango.
He tries to pretend that he isn't looking at the meal set out in front of him and refuses to touch it with various excuses - but in the end, he eats with great vigor.
He requests some okaka onigiri, but comments that they need more seasoning.
So... on the whole, everyone is saying that Sasuke acts much more like his first part self than his second part self in the novel - being very kind to the siblings and having many 'cute' scenes with them... those kinds of things. But that the novel is in general pretty dark, with people dying and stuff.