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Scott, the problem as I see it is you are looking at this all wrong. Think outside the box like when you write.
You are trying to bring your books and identify them into a system that is of our world, very one demensionally. This is just ass backwards. You need to be taking our world into yours.
Create your own system. For instance, create an alien type symbol and/or numbering system. (Sklorno, Kreterakian, or Quyth perhaps?) Blend them together to denote where they intersect each other. Then there isn't a dependency on sequencing like you are struggling with right now.
Think multi-dimensionally. Try not to look at time in a linear fashion but bend it and warp it how YOU want it to be. Identify them by where they intersect each other.
We're entering your world through your books, please don't try to bring your books into our world like this.
But, if you want a simple timeline like others have suggested here from Star Wars, meh.... I can certainly understand needing to dumb it down for mass consumption. Just my 2 cents.