“Of a trilogy” vs. “in a trilogy”

Question

I've Googled for a while now, and can't seem to find any good answers to this.

Which of the following is correct?

1. X is the second installment in a trilogy.
2. X is the second installment of a trilogy.

Answer 1

You should say:

XXX is the second installment in a trilogy of books.

Because XXX belongs to the trilogy and is in it.

On the other hand, you'd say of a trilogy if speaking of the trilogy as a whole, like this:

XXX marks the end of a trilogy.

Answer 2

They are both correct English. If you're speaking of the position of something in a sequence you use "of". For example,

The second of the three big snowstorms we had last year.

This usage can be extended to saying a book is the "second of a trilogy".

On the other hand, to say that a book is one of the members of a trilogy, you would say that it is "in the trilogy":

There are five books in the Hitchhiker's Guide "trilogy".

(You can't use "of" here.) So saying "second in a trilogy" is also correct.

This Google Ngram shows they are both used.