The Itachi Shinden novels are definitely worth the read. I can't say the same for Sasuke Shinden, though.

Itachi Shinden, as most of long-time Naruto fans may think, is just narrating how Itachi eventually got to the decision of killing his own clan. There's actually more to it than that. Not only does the novel dwell more on what Itachi was thinking at the time, but it also gives new perspectives to characters like Fugaku (Itachi's father) and even Shisui. How Shisui's family is doing and how he awakened his Mangekyo Sharingan is explained in the novels, for example. How Shisui died in the novels is different, too, and the way he died here is much, much darker. The two novels take you on a journey through Itachi's childhood, starting from when he had seen war at age 4 to when he was on his way to join Akatsuki at age 13.

Sasuke Shinden is... decent. It introduces new characters/antagonists, but overall, the story was pretty meh. It involved the Uchiha clan somewhat which kept it slightly interesting, but that was honestly the only driving point of the plot. The antagonist is your usual "people treated me wrong, now I'm gonna do the same thing to them" character. You really get to know about Sasuke's thoughts on everything here, though, that's one thing these novels nailed doing.