http://www.uesp.net/wiki/Skyrim:Sithis

As you can see, this book is presumably the beliefs of the Dark Brotherhood concerning the creation of the universe. However two things are off at the end. First, the line "Go unto the Sharmat Dagoth Ur as a friend." Second, the chant "AE HERMA MORA ALTADOON PADHOME LKHAN AE AI", similar to a chant in the 36 Lessons of Vivec. And with the book claiming that Lorkhan was created by Sithis, I believe I have found something. Dagoth Ur was sponsored by Sithis, via the Heart of his False Aedra servant. Does this explain his actions? Is the Tribunal related to Sithis as well? Do they know of this? Also, why would this book increase your Alteration skill?

I don't think that the Tribunal are connected to Sithis because they are enemies of Dagoth Ur. I don't see any connected between the Tribunal and Sithis other than this.

I believe that Lorkhan was simply imparting friendship between the Aedra and Ur because Lorkhan himself was being presented as an Aedra through the ideas he created. I can't tell if his representation of Dagoth Ur would be sinister or genuine though. I could be completely off with this because I've only just started looking into the topic ^^

The book was made before the Dark Brotherhood was (publicly) known to worship Sithis, when everybody still thought they were just a less-scrupulous Mephala cult. I always assumed it was sort of a Dunmer Creation Myth, since they glorify Lorkhan and scorn the Aedra.

The final message, "Go unto the Sharmat Dagoth Ur as a friend" seems fairly self-explanatory, to me: As Lorkhan went to the Aedra, arrayed in glory and presented as like-unto-them, so must one confront Dagoth Ur... whether literally, Nerevarine-style, or as a metaphor by the Tribunal Temple (where Dagoth Ur was their "Devil").

I can't remember what the meaning of the final words is... something about "secret knowledge is the weapon of change/chaos" or something like that, but the Lorkhan part is new.