Might sound pretentious but here it goes.
>It had had two windows…
I'd use only one had, not sure if correct but it sounds better imo
>stood up to begin…
you can always get rid of "to begin" or such words by simply adding an -ed to the next word or just getting rid of "to begin" entirely… I might not be explaining very well so here's an example:
"then stood up and pierced his scrawlings together" or
"then stood up, piercing his scrawlings…"
>the man did not sleep soundly that night.
You could start a new paragraph there.
The beginning of the next scene needs work spelling/grammar wise.
>He came upon, presumingly…
you can start a next paragraph there.
>The book appeared to be bound in human flesh, but the young boy was blissfully unaware of this at the time
If he doesn't know it then you could suggest it by describing how the book looks, instead of simply telling us what it is. Something like "The leather in which the book was bound was surprisingly soft with a skin-like oily texture to it, with a colour pigment similar to that of the old man's skin."
Ok I just noticed you changed the old man with the beard character to the young boy, so a flash back… But I'd still call him the "old man" as it's from his point of view and this seems a bit jarring.
>young lass
Where did she come from?
Consider giving the "old man" who turns into the "young boy" in the flash back a name at the beginning, as it becomes
a bit confusing when you at the german man into the mix
Cba to read the rest, consider putting the flashback of the young boy at the start since that sounds more interesting than the current opening page.