From the Oxford Corpus of English:
PER ANNUM
For 30 years, it had gone remorselessly ahead, at about 80% per annum.
An Elan bond, where the bondholder can exercise the right to be repaid in 18 months, currently yields 19 per cent per annum.
With this fresh impetus, the total edible oil processing capacity, including vanaspati, is expected to cross 20 million tonne per annum.
PER YEAR
We can currently get about 5% per year from investing in long-dated gilts, so we might aim to get 6% per year from the property.
Penetration of digital has hit 20 % in a year, with incremental revenue per subscriber at $22 per year and growing, he said.
This still leaves teachers $3,000 per year short of accountants, $17,000 short of computer systems analysts, and $25,000 short of engineers.
These example sentences are representative of the most common uses of these two phrases and, as one can see, there is no real difference between per annum and per year in usage.
As kiamlaluno says, per annum is traditionally used more in financial contexts than per year, but these sentences show that per year is also perfectly acceptable.
First of all, they are basically the same thing thing. However, I think there are some subtle differences in how they are used.
The first is more common and is probably preferred in most cases. It simply means "Tell me what that book is about, and be specific and accurate."
If someone decided to use the second phrasing, they would be putting more emphasis on the word "what" since what is being modified by the phrase 'exactly.' The phrase would probably come after the two people had been talking about the book for at least a sentence or two.
Because we are emphasizing the word "what", we just want to know very strongly what it's about!
What this would mean is "I'm really confused or unclear about what this book is about, so please explain!"
You can tell that "exactly" is emphasized in the first sentence because 'what' modifies it. So what we care about here is an exact definition.
You can tell that "what" is emphasized in the second sentence because "exactly" modifies it. So what we care about is getting an answer.
Best Answer
At any moment is the correct one. Or simply any moment.
In a moment means that something will happen very soon, literally any time within the next moment. This makes in any moment senseless.
At any moment on the other hand deals with moments as points in time. At any moment will mean, that at any one of this points something will happen. At a moment will be senseless, because you don't specify which of the moments you're talking about.