There are 2 ways to answer this.
First of all, technically speaking, the books themselves do NOT have canon levels assigned to them. As per Leland Chee, the keeper of the Holocron etc, etc, etc.., the Holocron is a database of facts, attributed to a source.
The source influences what the canon level of a fact is, but doesn't seem to have a canon level of its own assigned to it per se. You can only speak of a canon level of a work in a sense of "all the facts taken from this work will be listed with at-most this level in the database". You can call that a work's level of canon, if you wish.
By Leland Chee's definition, pretty much any non-novelization book licensed by LucasFilm defaults its facts to C-canon, aside from "Infinities" comics which were explicitly designated as "what-if" stories in TGFFA and are N-canon by design.
Having said that, let's forget precise terminology for a bit and give an example of a book whose facts are defaulted to C-canon but has several major N-canon details.
"Splinter of the Mind's Eye" is overall considered C-canon.
the Wiki page says so.
The canon rules quoted above say so.
I don't know of any statements from Leland Chee or LucasEmpire that it's NOT C-canon.
In addition, Lucas, in his preface to a SotME re-print (hell, the fact that he wrote the preface should speak volumes), placed "Splinter" on the same "stories inspired by Star Wars" as the other books:
Today it is an amazing, if unexpected, legacy of Star Wars that so many gifted writers are contributing new stories to the Saga. This legacy began with Splinter of the Mind's Eye
However (as the first book written after Star Wars: ANH, and before even Episode V: ESB), it was definitely bound to contain at least SOME facts and plot elements that was later contradicted by G-canon, and it did.
Luke has major romantic attraction to Leia, and she reciprocates.
During the lightsaber fight between Luke and Darth Vader, Vader states that he knows Luke is the pilot that shot him out of the way during the Death Star trench run, when it was Han Solo and the Millennium Falcon who fired the shots that disrupted the course of Vader's fighter.
Kaiburr crystal was supposed to increase one's Force powers. That was before Lucas decided to make the force more
As per Wikia: "The Kaiburr crystal was originally developed in the second version of Lucas's The Star Wars script. The script for Star Wars: A New Hope was a revised version of the fourth draft..."
In the novel, the beam generated by Darth Vader's lightsaber is described as blue, like Luke's, which may be due to the bland coloration of the lightsabers in the theatrical release of Star Wars Episode IV: A New Hope.
Luke fights Vader, and cuts off his hand.
As we can guess from original trilogy, Luke did not fight Vader until ESB Cloud City scene, and didn't encounter him again till Death Star 2.
This last one is not strictly speaking N-canon as we don't have 100% ironclad proof in either movie that no other Luke/Vader encounters ever happened, either before or after Cloud City, and there are ret-connable explanations for Luke being skilled enough to cut off Vader's hand (posession by OWK's Force Ghost is most popular).
NOTE: some of these bullet points were from Wiki/Wikia, though not all.