My view is that, the book contains excellent guidance, but maybe there is a need to speak of this important teaching in a way that will be understood more widely and popularly, so maximum number of souls can benefit. And there the Catholic Teaching on Treasury of Merits can be helpful imo.
Our Lord Jesus said in Gospel of St. Matthew Chapter 6:19 "Lay not up to yourselves treasures on earth: where the rust, and moth consume, and where thieves break through, and steal. 20But lay up to yourselves treasures in heaven: where neither the rust nor moth doth consume, and where thieves do not break through, nor steal. 21For where thy treasure is, there is thy heart also." and Catholic Tradition has always understood this, as the Council of Trent explains, that by good works we lay up eternal treasures.
So, for e.g. just an estimate, let us say, it takes 30 years of habitual living and growing in the state of grace, without mortal sin, going to Mass and Communion daily or frequently, loving God and neighbor more perfectly, and practicing many other spiritual exercises, to advance one stage.
So, when we are first baptized, all our demerits are taken away. But our merits are just beginning to grow, we start from zero. Each good work done in grace increases our merits and the degree of grace - and corresponding degree of glory in heaven - we attain to. So, let us say, as an example, that it takes 10 Million Merits to advance to the next stage. Supposing every Rosary we pray, every Mass we assist at, every Holy Communion we receive devoutly, every prayer we say, or good work God gives us to do, increases our merits, if we do it all very well, gives us 1000 Merits daily. Then, in about 30 years or 10,000 days, if we live like that daily, we will go to the next stage.
Of course, mortal sin destroys all our merits. Then, we have to start from zero again, even after Confession; unless, we have very great tears of contrition for former sins, as some of the Saints did, and also do much penance for them. So, key is to avoid all mortal sin.
As Scripture says, our life is short, and on average is 70 to 80 years. Thus, even in 80 years, it will be hard, but it can be done, to try to complete stage 3. Of course, as the St. Theresa says, if we put greater love in our prayers and even small acts of love, we can grow faster.
Thoughts and comments from people here?