What does an exclamation mark inside parentheses "(!)" mean?

What does an exclamation mark inside parentheses "(!)" mean? I saw this in a book review I was reading and I didn't understand what the use of it was. Can someone help me?

This usage is simply a passing note of astonishment at the preceding word (or perhaps phrase). Typically there will be no further comment on this astonishment, and it is just an aside. This may express real or feigned (sarcastic) astonishment by the writer, or be used to call attention to a surprising aspect of the use of the preceding word.

It would help to see the example you're referring to, but this usually is an informal way of emphasizing -- usually with sarcasm -- some piece of information that comes in the middle of a sentence rather than emphasizing the whole sentence. For example, if I said

I saw this in a book review I was reading (!) and I didn't understand what the use of it was.

The exclamation would apply not to the fact that I didn't understand some aspect of a book review, but rather would sarcastically point to the fact that I was even reading a book review.

(!) is the written equivalent of a brief pause to raise the eyebrows. It expresses alarm or astonishment at whatever was revealed by the preceding word.