It's unclear who wrote the Elder Scrolls. They may not have been written at all.
From what I gather, the scrolls are simply more a consequence of existence than a work of authorship. The Gods themselves are often intrigued and impressed, even fearful, of the scrolls. We know from Oblivion that the power of an Elder Scroll can even undo the workings of the Daedric Lords.
So... they seem to be more a part of reality. Leftover chunks that express all things, all times, all places. They are a part of creation, but they were not created. They simply exist.
And, as often as not, they don't.
http://www.uesp.net/wiki/Lore:An_Accounting_of_the_Scrolls.
You can see, sometimes they Are and sometimes they Aren't. That's the fascinating thing about them.
I rather think of 'em as pieces of existence itself. Reading the scroll gives you knowledge of existence, which can be past, future, or present. Utilizing the scroll changes the existence itself, allowing you to do things not supposed to be possible, like undoing a Daedric curse, or warping time