A mistake in the sentence is this: to us understand.
What is the mistake?
The mistake in the sentence is that the phrase, " to us understand" is wrong. The sentence should be written correctly as follows; "The book was too complicated for us to understand."
By rewriting the sentence in this way, we can see that the confusion is removed and we now know that the complicated nature of the book affected the narrator and other persons. The preposition, 'for' makes the sentence clearer to the reader. So, the mistake is "to us understand." Using the correct grammar makes information clearer to the reader.