I’ve preached an introductory sermon on four books: Genesis, Ecclesiastes, 2 Corinthians, and Philippians. When you asked the question above, I opened up my sermons to see what helpful answers I might be able to give.
I tried to show people a macro view of the book, by showing its location in the canon, summarizing the message of the book as a whole (this helped me get a title for the whole series), giving an outline for the book, and explaining background information on the book (author, date, etc.).
So for Genesis, I talked about the book’s importance for the Christian life, showed them Genesis’ location in the Pentateuch, talked about the whole story of the book and gave them a one-sentence theme, gave them background information, and concluded by talking about Christ in Genesis.
For Philippians, I spent more time on background: on who the church was and its founding in Acts, who Paul was. I talked about the Pauline epistles a little bit, and I concluded by describing Paul’s purpose in writing Philippians and its central thread (the gospel). My introductory sermon on 2 Corinthians had a similar outline, as it is also a Pauline epistle.
For Ecclesiastes, I began with introductory information (author, date, and genre), and then tried to summarize its theme by reading key verses, trying to show them the darkness and difficulty of life under the sun, and concluded with the key verses of 12:13-14 to bring the entire book into perspective.
One of my favourite things to do in preaching is to give people a big-picture view of a chapter of Scripture, or of a whole book. That is one of the greatest benefits of preaching an entire book at one time, I think.
But that benefit provides the main difficulty, which is mastering the message of the book as a whole and communicating it! But it is worth it, because you can’t preach a whole book accurately without grasping the big picture yourself.
A helpful resource (which I didn’t use, but which I would love to use next time!) are Mark Dever’s books that include one sermon on each OT and NT book: Promises Made and Promises Kept.