Find the Imperial Library through the ES homepage - Links / Fan sites ... then search. Reason I am cautious here is that some of this came from other players ... and some from the books I read.
There is stuff that I have never read from Arena, Daggerfall, Battlespire and Oblivion itself, plus new bits (some from Devs in this Forum and the Imperial Library and it all keeps getting retroactively changed)
I go get link for your first TIL visit: (hmm - search for references to Yffre and Jephre drew a blank at TIL - but here:
http://www.imperial-library.info/pge3/valenwood.shtml
PgtE 3rd edition
The Wild Remain
And yet there are signs of other forces stirring. The Wild Hunt has been sighted for the first time in over five hundred years, although to what purpose is not yet clear. Falinesti, the "walking city" of trees, has rooted itself for the first time in recorded history. A new Bosmer prophet has arisen, known only as the Precursor, who preaches that the old Forest God Y'ffre is returning with new gifts for his favored people. Whether these are isolated events, or signs of some great change coming to Valenwood, remains to be seen.
If you read a Dance in the Fire then there are references there to the Green Pact - a treaty between the Bosmer and Yffre that gave the Bosmer Yfrre's protection and patronage in return for the Bosmer's agreement not to eat the plantlife of Yffre's Forest - and there was more to it.
You will need to spend a fair amount of time to winkle out enough to begin to aussage your apetite for new Lore though.
Elhnofey
http://www.imperial-library.info/fsg/abrasivenutarticle1.shtml
This is a link to Development of the Species
And has some content on early times
add in a section arrived at from bittergreen's link above:
Y'ffre
(God of the Forest) Most important deity of the Bosmeri pantheon. While Auri-El Time Dragon might be the king of the gods, the Bosmer revere Y'ffre as the spirit of 'the now'. According to the Wood Elves, after the creation of the mortal plane everything was in chaos. The first mortals were turning into plants and animals and back again. Then Y'ffre transformed himself into the first of the Ehlnofey, or 'Earth Bones'. After these laws of nature were established, mortals had a semblance of safety in the new world, because they could finally understand it. Y'ffre is sometimes called the Storyteller, for the lessons he taught the first Bosmer. Some Bosmer still possess the knowledge of the chaos times, which they can use to great effect (the Wild Hunt).
And the references that they draws from ... etc
Hope you read the Monomyth - that highlights the differences between human and elven beliefs too. Really it is a very amorphous mess ... great fun