I can try to provide an interpretation, though I can only state the barest of concepts explored with any degree of certainty.
The barest concepts:
The letter, from the 5th Era, is a warning to those just transitioning into the 4th Era about the dangers of Landfall. The Landfall is Baar Dau (the Ministry of Truth moon) finally fulfilling the mission it was tricked into doing and catastrophically smashing into Vivec City. The destruction from this event is implied to have utterly destroyed Morrowind; the survivors (likely miners) now live an almost complete subterranean existence, as the surface is inhospitable. The letter further accents the importance of love, and its role in preventing said catastrophe. "Know love to avoid the Landfall."
There's also the phrase, "I am and I are all we," which ties into the Godhead and the whole universe and every being contained within being one of countless multiple personalities of a fractured, schizophrenic dreaming god-mind.
Now, to enter things of a more shaky explanatory nature:
We are given a written account of the progress of creation, highlighting a pattern of gradients or subcreation; echoes, if you will.
Void to Aurbis, naught to pattern.Aurbis to Aetherius, possibility to maintenance by time.And so on, and so forth. All of these progressing gradients form a continuing pattern of sorts (subcreation must continue) that lead to the centerpoint of Mundus:
C0DA translation: if all previous gradients continue along this path, especially given that there is now a centerpoint, impossible Mundus, the process of continuation can be pre-figured. And a continuation of that pattern:
Mundus to Mortal Death: centerpoint to the soon recycled.But what subcreation follows mortal death? The Loveletter cites many examples of attempts to reach it:
You in the Fourth Era have already witnessed many of the attempts at reaching the final subgradient of all AE, that state that exists beyond mortal death. The Numidium. The Endeavor. The Prolix Tower. CHIM. The Enantiomorph. The Scarab that Transforms into the New Man. Why did Lorkhan fail CHIM?
“The Lunar God failed by his own devices, to show the new progeny how they might not.”The last and final subcreation, then, can be described as follows:
Those who do not fail become the New Men: an individual beyond all AE, unerased and all-being. Jumping beyond the last bridge of all existence is the Last Existence, The Eternal I. As Vehk's Teachings put it,
What is the Tower’s secret?
How to permanently exist beyond duplexity, antithesis, or trouble. This is not an easy concept, I know. Imagine being able to feel with all of your senses the relentless alien terror that is God and your place in it, which is everywhere and therefore nowhere, and realizing that it means the total dissolution of your individuality into boundless being. Imagine that and then still being able to say “I”. The “I” is the Tower.
The loveletter is not just a warning to the 4th Era about the catastrophic effects of the Landfall. It's also Vehk's scripture of Love, and it introduces a fundamental shift of the way the inhabitants think about the universe which they reside in.
There's also Lunar Currency, but I know way too little of it to even attempt a proper explanation.