From my perspective, this book is useful to young men in their 20's looking to take charge and get on in the world - while doing so in a Godly way. I think the utility spans to both the secular, and non secular audience and therefore can be useful to both parties, but the book is geared to a Christian audience.
I enjoyed the authors writing style, and his personal anecdotes were funny and insightful, so the book was an enjoyable read while being educational.
Personally, as a young businessman who is married, the book resonated with me and left me with a greater zeal to diligently work and serve God, both by action in my business, and by giving to the Kingdom effort with my harvest; while also allowing me to recognize shortfalls within my "ability" living out the role of a Godly husband in my marriage.
In particular, Manly Dominion helped me see that in my marriage I was not leading firmly. I could see that my headship was more "passive" than not, and what I saw as authority was really demeaning and demanding, and more boyish than mature.
I think the book highlighted well the general Christian ethos in marriage, that the Man is to lead with assertiveness, but very much in a spirit of servitude and love - you can not do this aggressively, which is why I think "dominion" mindset (which I think what the author means is being intentionally decisive and proactive) is the proper Godly approach.
On a spiritual note, the book helped me see a bit more clearly the whimsical nature of a lot of Christian thinking. This is outlined very well in the third part of the book on "Manly Dominion in Decision-making". What I gathered the author had, was a balanced perspective that still acknowledges the Grace and power of God, but illustrates how we are still to move forward and make decisions within our own framework. Ie) You are still to pray, meditate upon the scriptures, rely on God, but also you are to use your rational faculties and plan and get a move on. I would think its best summed up as working in partnership with the Lord as you move forward in Dominion.
From a personal note these are the things that had the greatest impressions on me, and there's also lots more of value within the book. If somebody want's to read up more on Christian masculinity this is a good read.