Writing the introduction to a book is sometimes the best part. For this reason, it is also often enough the only part of a book that gets written. For this, and other important reasons, it is a good idea to save it for last. Writing the introduction after the book is complete is good not just to keep you from losing interest after writing the fun part, but it also keeps the content of the intro consistent with the intent of an intro.
Introductions are generally used to provide an overview of the book, and the overview to a book is typically most efficiently done by waiting until the book is actually completed.
Introductions are also places where some authors will take the time to tell the reader what they think, in a way, or to a degree they have not done in the book itself. This is a common practice in non-fiction books written by journalists, for example. Journalists tend to write books that aim to be objective, and so the introduction becomes the place where they can write their opinion.
Introductions, though, more than anything else, should give the reader an idea of what they are in for. Some introductions provide a chapter- by- chapter or section- by -section review, covering the highlights and perhaps a key detail or two.
A book introduction is not unlike an introduction of friends (though the book introduction is certainly longer). The key is to convey a sense of the content, and the manner in which the content is being handled.