At the moment, you can access the Arceuus Spellbook very, very early on, thanks to the fact that Favor was removed. I believe all that's needed is access to a POH with an Occult Altar, or for Ironmen, getting to Kourend. I'm not sure, exactly, 'unlocks' the book for use.
I know you don't need to complete Ascent of Arceuus or A Kingdom Divided to unlock it, as I haven't yet. I have access to Kourend, that's it.
The only reason I bring it up is that I'm barely an intermediate player - my main is only 1200 total level - and man, a lot of stuff in the Arceus spellbook confused and rattled even me. I can't imagine how much trouble it might cause a genuinely new player.
Also, Ironmen are like, weird snowflakes (no offense to Ironmen), and I guarantee you they're going to get fussy about people having access to it in unusual circumstances.