**** PLOT SPOILERS ****
OK, so Nar the insane dorf is suddenly going through books, which he has never done before, and that is alarming. If there was any connection between the book-rummaging (which was used as a plot device *twice*), I missed it in the dialog which "resolved" the mystery.
The middle part of the book - with Wadu's family and the attack on Nar - was pretty much filler and also made no sense. We learn at the end of the book that Saruman has some kind of underground path and has met with Nar many times. If so, why does he need to send an army to get him? Why not just meet him again, do some Wizard-y stuff, and spirit him off to Isengard? And why is Nar worth abducting at all? Saruman has clearly manipulated him, and convinced him of only what he (Saruman) wants him (Nar) to believe. He is no danger to Saruman, and no use to the Rangers or anyone else. He knows nothing of value to Saruman, either. The only plausibly interesting thing about him is that he has talked to Saruman, and that only came to light because Saruman drew attention to him in the first place!
Second, the whole "resolution" makes no sense. The original mystery was how Nar knew of Elrond's words. The answer to that is supposed to be that Saruman gave him that information. OK, so how did Saruman know? If he is somehow able to eavesdrop on Elrond in Rivendell, there is virtually nothing he *doesn't* know. According to the lore, even the *existence* of a living heir of Elendil was a closely guarded secret, and Sauron's discovery of it (when Aragorn used the palantir) was so alarming that he scrapped his existing plans to launch an *immediate* attack on Gondor.
Saruman could conceivably have learned about Aragorn independently. Gandalf might have disclosed it to him, before Saruman unmasked himself as a traitor (although I don't think anything in the lore suggests that he did). Also Aragorn revealed his true identity to Eomer when they met, and Eomer reported this to Theoden, meaning Wormtongue was aware of it (during the confrontation in the Golden Hall, Wormtongue mentions other information Eomer reported). And Wormtongue would have no doubt faithfully reported it to Saruman. But the meeting of Eomer and Aragorn happened only a couple days before the Battle of Helms Deep - not much time for Saruman to plant the information in Nar's mind, and one would suppose he had far more pressing matters to attend to around that time...
But even that doesn't explain Saruman's clairvoyance regarding the words of Elrond - or why Saruman would even care. The Oathbreakers were sworn to help *Gondor* fight *Sauron*. Saruman's immediate plans were for the destruction of *Rohan*. Having accomplished that, he could have prevented Aragorn from even reaching Dunharrow and the Oathbreakers.
So none of it really makes any sense. A recurring theme in the lore is that the malicious, power-mad villains like Sauron and Saruman are so focused on their own schemes, and so confident of their success, that they are blind to the weaknesses that will ultimately be their undoing. It is pretty hard to reconcile that with Saruman: a) knowing about the Grey Company, b) somehow knowing the exact words of Elrond, and c) diverting his own attention from the war he is about to launch to elaborately manipulate the mind of an insane dorf for no apparent reason and d) diverting his elite forces (Uruks) away from those battles to capture that insane dorf, again for no apparent reason. Especially given that, contrary to the inventions of the film, the Grey Company does not arrive until after the decisive battles between Saruman and Rohan.
A more lore-accurate attitude by Saruman, even *assuming* he knew they were coming, and *assuming* he somehow knew the words of Elrond, would seem to me: "Hah, the fool Elrond sends a score of rangers to contend with my army of ten thousand! But they will be too late! Let them come! Once we have overrun Rohan and taken captive those who are not slain, the Shire, Breeland and the rest of Eriador will be defenseless! Bwa-ha-ha!"