A "coherent" is not the same as "comparable to non-self-published authors".

Early LLMs were trivially non-coherent. The stories it wrote should shift constantly. The story could start on the moon, then the character could drive to Madison, Wisconsin.

A more obvious way to see this is in video generators. If the "camera" turns 180° twice, we often look at a completely different scene.

Can current models write an entire coherent book? I don't know. I have never wanted to generate such a book. But "do I like this book" is not a good test for coherence.