The book would thus draw a limit to thinking, or rather – not to thinking, but rather to the expression of thoughts: Because in order to draw a limit to thinking, we would have to be able to think both sides of this limit (we would thus have to be able to think what cannot be thought).
So the limit can only be drawn in language and what lies on the other side of the limit will simply be nonsense.
The last words of that sentence are einfach Unsinn. Literally they mean ‘simply nonsense,’ not ‘simple nonsense,’ which would be einfacher Unsinn. There has been some disagreement about this, and much discussion, but it is cleared up in Nordmann, p. 82, note 48. That is, how these two words should be translated is cleared up there. What to make of them is another matter.