(Book 1): Your adventure begins at dawn. As long as the sun is shining, your journey should be a pleasant one.

(Book 2 - 8): Your adventure begins at midday. Keep a steady pace and you'll come through just fine

(Book 9 and beyond): Your adventure begins in the dead of night. Your road won't be easy, but a rising sun awaits your journey's end.

Bonus points if you get the reference.

Yeah, I don't get it. Most books so far except Urohringr, Utaan and Ofolrodi started with the morning sun.

I don't know about the relevance to Austraeoh (I am still in last third of Utaan), but the reference is to Kingdom Hearts 1st' difficulty settings, which most notably adjust how you level up in the game. Dawn is fast leveling / easy until level 50, slow and difficult afterwards. Dead of night is vice versa. Midday is the compromise between the two. Note that the sequence Dawn / Midday / Dead of Night does not directly translate to easy / intermediate / hard.

That said, generalizing 2-8 like that sounds a bit iffy to me, or there is something I am missing here. If there is one Dawn book, I would say it is Yaerfaerda (because it's a foregone conclusion how things will work out until the Grand Choke, which is an everlasting trek of lethal uncertainty), and possibly Austraeoh and Ynaanluutr qualify. Innavedr (disregarding its beginning) and Odrsjot (disregarding its end) were closest to steady pace-like. Eljunbyro, Urohringr and Utaan (a bit difficult to generalize due to its sheer length) seem like Dead of Night books to me. This paints an interesting symmetry for the same-starting-vocal-books, with U starting in regret and misery, Y being essentially "we know what has to happen for most of this book but we have/had no idea where it will end up"-books, and everything else being neither here and there.

I don't feel like I am hitting the nail on the head here, so I would appreciate some elaboration.

Even though meant for austraeoh, it is also a parable of life, assuming you believe in something after for the last part.