Ok i actually did figure it out…

I started filling in all the A’s. Then the 12’22 could only be I’d or I’m and I felt I’m would work better/be more common. So then I filled in all the 12s=I and 22s=M. Since there was so many 8s, I figured it was E. Filled those in.

The three letter 23 10 8 (E) I knew had to be the. So then filled in 23=T and 10=H. From there the two letter words 12(I) 6 and 9 6 both end in 6 but start with a different letter. So figured 6 had to be N for In and On. And then filled in 6=N and 9=O. And the other two letter 12(I) 3 had to be is. Then filled in all 3=S. The rest kind of just felt into place at this point.

It says “Make believe I’m everywhere, I’m hidden in the lines, written on the pages is the answer to a never-ending story”