Its a fact longknife that the Elder Scrolls series is filled with contradictory books on various subjects. Just like real life is filled with contradictory books on various subjects. You can either accept the fact the books in-game are designed to be falliable just like in real life or you can find another game series where everything told is at face value, your never lied to and nothing requires more then a casual thought. Why does the book exist to make you look stupid for believing the word of a vampire when he's talking about vampiric abilities. I sincerely hope this scene is in the DLC.
Dragonborn: "You don't seem like the Volkhar i've read about..."
Volkhar Lord: "You mean that book Immortal Blood, the author was a vampire and you believed him where are your brains in your fangs?"
It's not contradictory if there's nothing to contradict it.
Also, let's assume for a second you're right and the book is false. There's no evidence then that the author is actually a vampire at all. It's anonymous, and if you think it's foolish to believe the content in it (if written by a vampire) is true, it's equally foolish to believe that the book was actually written by a vampire if the information he's given is false.
There is opposing opinion, Immortal Blood makes the claim that the Cyrodiil strain is unique in its ability to blend into society. This however is proven absolutely false from the earlier game daggerfall where we had ninr vampire clans all of whom could hide and walk among human society. That being said when the lone authority IS a vampire you never should have trusted it to begin with.
Not true. Immortal Blood doesn't say anything about the Cyrodiilic bloodline being unique in its ability to blend.
"I told him what I could. There was but one tribe in Cyrodiil, a powerful clan who had ousted all other competitors, much like the Imperials themselves had done. Their true name was unknown, lost in history, but they were experts at concealment. If they kept themselves well-fed, they were indistinguishable from living persons. They were cultured, more civilized than the vampires of the provinces, preferring to feed on victims while they were asleep, unaware."
It doesn't say it's an unique ability, only an ability they are highly skilled in.