I've Googled for a while now, and can't seem to find any good answers to this.
Which of the following is correct?
- X is the second installment in a trilogy.
- X is the second installment of a trilogy.
You should say:
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:
They are both correct English. If you're speaking of the position of something in a sequence you use "of". For example,
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":
(You can't use "of" here.) So saying "second in a trilogy" is also correct.
This Google Ngram shows they are both used.