But it's not in Immortal Blood, so apart from the info on Cyrodiilic Vampires we have no information about other Vampires mentioned in IB that has been confirmed outside the book (although I haven't read it in a while, but don't think any Daggerfall Vampires are mentioned in it either). My speculation was that all of the types of Vampires that we 'know' are not mentioned (apart from the Cyrodillic).
To speculate even further (into somewhat ludicrous and unsupported territory), perhaps Immortal Blood could be a test designed to recruit only those Vampires astute enough to see the hidden 'seek out The Order' message. Someone may have deemed those who discerned the truth in the book to have been worthy to join The Order. For some reason I am envisioning solo Vampire pilgrimages to every temple with a priest in Cyrodiil, searching for the author.
The Order are my favourite Vampires from TES (although I probably played longer as a Quarra), and the idea of long term game playing (like the premise in the film http://en.wikipedia.org/wiki/Razor_Blade_Smile is one that has always appealed to me, but unfortunately when I played a Vampire in Oblivion I had a bug where going through a door which loaded a new cell turned me into a male.

I just couldn't find a way to role play that, so sought a cure for the Vampirism.

I believe the other clans(Vvardenfell, IIiac Bay) weren't mentioned because Bethesda must have felt we already knew about them. Same reason why they aren't mentioned in Manifesto Cyrodiil Vampyrum and other clans like the Whet Fang and Glenmoril Wyrd, are. The only thing Immortal Blood got correct about the "Volkihar" is the name and their cruel tendencies.
Immortal Blood also served to explain the Cyrodiilic Vampires, as did "Vampires of Iliac Bay," serve to explain about the Montalion and the other clans, likewise "vampires of Vvardenfell," served to explain about the vampires of Morrowind in each of the games. Bethesda must have been lazy to continue the tradition and thus re-release Immortal Blood as a staple vampire book in Skyrim.
You may be right on that regard. Using the book as a message under the masquerade of a work of fiction to entertain mortals. Sounds brilliant.

I always liked the Order. Their tenets are wonderful, and they are everything many people dislike; hungry for power, money lovers, manipulative "monsters,", deceitful elitists and politicians, dark cultists infiltrating society as regular people. Reminds me of Palpatine/Sidious(from Star Wars).
? I have been saying all along there are at least two and likely three vampire clans in Skyrim?
Who is not reading who's posts now?
I said that Morvath and his gang are Order vampires as we know from Immortal Blood that that is his lineage.
We also know there are Volkihar as they are the ones the game specificially names so.
I speculate that Harkon and his court are a seperate strain entirely, purebloods.
However, no matter who belongs to what strain, their game mechanics are the same.
All vampires have the same (set of) looks, the same abilties and weaknesses.
Aside from the PC the only NPC that ever becomes a vampire lord is Harkon, and only on two scripted occasions.
Gameplay isnt lore, you cant deduce from looks or abilities who belongs to what strain.
To Abrinth: I love your reasoning to explain why Immortal Blood was written.
Oh how I love pushing your Sesame Street button's. Of course, I simply said that because you made an attempt to compare me to the uneducated children for posting content from the UESP(which by the way, had more clans listed other than JUST the Volkihar.)
I pretty much agree with many of the things you said, but what make you think some of the vampires in Harkon's court are all purebloods? I'm just curious. I think some are halflings, like the player.