I consistently find Old to be an illuminating resource on all things related to Reformed worship. This book was no exception, as he mines the Old and New Testaments (as well as the second-third Christian centuries) for insight into the theology and practice of preaching. His perceptive analyses of canonical sermons (which are usually abridged) are as exciting as they are informative. Here are just a few insights:
• The practice of having a proposition developed in three or four points has incredibly deep roots, going back into the biblical era.
• The principle of Scripture interpreting Scripture has always been crucial to preaching, with many practitioners utilizing a secondary text as a key to opening up their primary passage.
• In the early church, it was not typical to separate evangelism from the regular preaching ministry -- the call to believe the gospel was constantly preached to the regular congregation of the faithful.
There's much more to find here; these are just off the top of my head.
Some evangelical readers may be put off by Old's ambivalence about the authorship of some books of Scripture (especially in the OT, where he's perfectly comfortable taking higher criticism at face value), and perhaps bewildered that he still insists on the inspiration, authority, and historical fidelity of the biblical text itself. Occasionally he'll make some practical application of the development of a book by the Isaianic school or what have you; I generally found these applications helpful, even when I disagreed about what he based them on.
On the whole, this book probably belongs in my top 3 books on preaching. We're I ever asked to teach a course in homiletics (God forbid, for the sake of the poor students), I'd probably assign this one alongside Preaching by Tim Keller and Preaching in the New Testament (NSBT) by Jonathan Griffiths.