I'll begin by saying that the book is a good idea.  Relationships take work, and that's something easily forgotten in a culture that is daily being conditioned to think that emotions are what defines a relationship, and that if you just don't feel it any more, it's time to move on.  Life is too short, you've got to take care of yourself.   Of course, we don't address the issue that, with an attitude like that, it's pretty much guaranteed that sooner or later, you ain't gonna feel it.

One of the loose premises of the book is that if we put the same effort into maintaining and rekindling our love with our spouse as we did in wooing them, our relationship would, most likely, be better than it is now.  We all tend to get a little lazy after years of marriage.  It's easy to take the other person for granted, to get bogged down in obsessing about their shortcomings rather than their strengths, and to focus on our own needs that we don't feel are being met adequately, rather than focusing on how best to meet our spouse's needs.  This is a very valid premise. 

Towards that end, the book begins with very practical tips - in the form of daily 'dares' - for beginning to lead the heart by example and intention, rather than allowing the heart to rule us and determine what we will or won't do.  Heavy emphasis is placed on self-sacrifice, on not attempting to change the other person, but rather to change ourselves and our attitudes towards them.  Love is a decision, the book emphasizes, and that decision to love the other person shouldn't change based on whether or not the other person returns the favor.  Of course, the underlying assumption is that if you begin to change yourself, the spouse is likely to respond positively.  And as you get farther into the 40-days, there are more and more daily 'dares' that directly involve or require the spouse in some fashion.  

This is fine, although the movie Fireproof would seem to contradict the book on several levels.  The protagonist goes through the 40-day journey, but it's pretty clear from the movie that he wasn't able to complete all of the days.  Perhaps as many as half of them, frankly, because his spouse is not reciprocal.  

As such, the book develops sort of a split personality.  On the one hand, it has practical little suggestions (challenges) on how to change our thinking to be more positively focused - like it was when we were dating or courting our spouse.  We are challenged to regain those charitable feelings and actions that once came so naturally - so eagerly.  This is good, practical, reasonable stuff.  The dares created by this personality of the book are for the most part doable - if somewhat vague.  Perhaps some more practical directives could be given for the truly clueless, but overall, they're good reminders of how we need to be and what we need to do, if we ever hope to change how we feel.

The other personality of the book is evangelical.  Around day 20 of the challenge, the dare is to give your life to Christ.  This may not be a big deal for some people, but it's a rather large step for others, I'm guessing.  The same with getting into a daily Bible reading habit, and a daily prayer habit.  The movie conveys the message more clearly than the book - the most important issue to get straight in your life is a relationship with Jesus Christ.  EVERYTHING else is secondary.  Faith is not a means to an end, a method for convincing God to save your marriage (though it's implied rather strongly).  Rather, faith is the end in itself.  Everything else will get sorted out in the wash.

The book is less clear about this.  It makes no bones about being a Christian approach.  It attempts to provide Biblical support for all of the various ideas that it gives over the 40 days, whether the ideas a purely a common sense sort of thing, or a more specifically spiritual dare.  But the book could and should be a lot clearer in stating that the most important thing to come away with is not an improved or saved marriage, but rather faith in the saving forgiveness we receive by God's grace through Jesus Christ.  And if that's the most important issue, and if the book really believes this, it seems kind of odd to get into this half-way through the 40-days, and then use this as the basis for some pretty monumental steps and life changes in the last 20 days.  

The final struggle I had with the book was the legalistic tones that would creep in at times, particular in the last half of the 40-day journey.  We're exhorted to love unconditionally.  My understanding of sin says that this isn't possible.  Sure enough, one or two days later, there's a section talking about how we can't possibly do this on our own, but only by the power of the Holy Spirit.  Except that, even with the Holy Spirit, I don't know anyone who loves perfectly and completely unconditionally except Jesus.  We're all sinful.  We're all selfish to varying degrees.  And at times the book holds up expectations that I don't believe we are capable of, setting folks up for some painful and dangerous failures.  

I also find it interesting that the 40-days don't encourage anyone to go to church.  Commit your life to the Lord, but don't plug into a community of faith?  How American is that!?  In fact, practically no mention is made of anyone outside of the husband and wife in this process, except for one of the latter dares in the book is to find a marriage mentor - an older couple who can serve as relationship mentors.  Great advice, but if you're going to drive home the point about a relationship with Jesus, why not dare people to find a church where they can be nurtured in the faith?  I think this whole idea was mentioned maybe once, towards the end of the book.  I would have preferred it be more central - but then again, I have a pretty high view of worship and the role of worship in our lives - not as something that we do, but rather as an opportunity to receive God's grace and forgiveness.  

This book has some very good things in it.  And I will undoubtedly recommend that couples give it a try (I'm even planning on leading a study with it).  But it struggles between being a self-help book, and being a book that reminds us that self-help is an oxymoron, and that the help we ultimately need most is the forgiveness of Jesus Christ as we receive His gifts of forgiveness and peace.  The movie struggled with this same dichotomy between you should be the change you want to see, and nothing matters but Jesus.  In the context of Christian community, and with some wise leadership, these issues can be addressed, and the good that is in the movie and the book can be maximized, while hopefully avoiding some of the emotional and spiritual pitfalls taht might otherwise snag folks.