Decent summary of them: http://www.imperial-library.info/content/facts-and-opinions-nu-hatta-intercept
Key line:
Here's the kicker.
These letters are trying to warn the Empire that an Oblivion invasion is imminent but Ocato wouldn't take it seriously. He says that the Emperor and Empire have more blatant threats to worry about at the time being.
The big point of these letters however is the role of Towers and Stones.
What I found most interesting was that the Heartland High Elves, the Ayleids, built White-Gold Tower to mimick Ada-mantia/Adamantine/Ur Tower.
The Altmer of the Isles preferred dracochrysalis, that is stopping magical energies from degrading into lesser forms, whereas the goal of White-Gold Tower and the Ayleids was to re-reach the divine by reversing the effects of Convention.
The letters blatantly point out that the Ayleids were not destroyed, that they just withdrew into a hidden existence. The intolerance towards the former slaves of the Ayleids (humans) drove a lot of them to Valenwood.
Also, White-Gold Tower was built to mimick the Wheel that represented creation.
I can't help but believe that the extremist Thalmor government (who we can all commonly agree want to do something to the Mundus, either undo it and return to Divine Matter, or unmake men, or whatever) that reformed the Aldmeri Dominion are in fact the Ayleids driven from Cyrodiil. The Intercept letters say that it was a conspiracy in the making since the Merethic Era, and alludes to the conspiracy stopping at the Oblivion Crisis... but we never saw the return of the Ayleids in Oblivion? Why was that? Or was the Oblivion Crisis part of a plan to weaken the Empire's strength and give the Thalmor boasting rights for "stopping the invasion," which we know is total bogus. Maybe they closed some gates in Elven lands, but we know that it was Martin with the PC's help who sealed Oblivion away.
The first Thalmor government formed in Valenwood but was dismantled (was it?) when Tiber Septim brought Valenwood under the Empire's control. The second Thalmor government popped up in Summerset Isles and claimed to save the Altmeri people from the Oblivion Crisis.
Is it possible that the Thalmor are in fact Ancient Ayleids (Ancient Aliens, bro, ancient aliens) that deal with the Principalities of Oblivion in order to utilize the towers for their own twisted purposes? From the Intercepts:
If the Ayleids made their own Wheel within the Wheel, were-web aad semblio, what would happen if they plucked its strings?
So either the Thalmor are Ayleids that have merged into Bosmer/Altmer culture through breeding and other means over the centuries and kept their goals, or the Thalmor really studied the Ayleids to find out what the purpose of White-Gold is, and their ultimate goal is to use White-Gold's resemblance of creation to "pluck its strings" to try and end it all. Either way, the "return of the Ayleids" cited in the Intercepts never happened in Oblivion, so I think Bethesda has something planned for them.