The book is vintage Beeke. While not a collection of independent essays, most of them can be read independently, although the essays on the Erskines build on each other. In this book the reader gets an outstanding (if sometimes limited) perspective on Reformed and Puritan spirituality. Aside from some editorial hiccups, this book is a treasure.
The flow of the essays moves from Calvin's piety, which involves the work of the Spirit through proper channels, to England, where we find Puritan spirituality. Beeke takes a lengthy detour in Scotland to discuss Boston and the Erskines. From there we sail to Netherlands and conclude with comparisons between Puritanism and the Dutch Second Reformation.
Key themes are assurance and godliness. Beeke clears up several myths about Puritanism but he does highlight, if only briefly, areas where the Dutch Second Reformation really couldn't keep it together without going full pietist. Some examples on content:
Calvin on Assurance: The Holy Spirit is the bond that unites us to Christ. Excellent opening by Beeke. Places the internum testimonium Spiritus within the larger working of salvation, and not just as it relates to Scripture.
I do appreciate the fact that Beeke notes several of the 18th century Scots did *not* hold to the Covenant of Redemption. Sometimes there is a tendency to think all of our heroes are united in these points. John Brown and Thomas Boston, by contrast, knew what the Covenant of Redemption was and collapsed it under the Covenant of Grace.
This book is eminently useful, though not every essay. The essays on Teelink and Frelinghuysen are helpful, if only to see what happens when one goes full pietist (though of course, that was not Beeke's intention).