Hi Colin. I searched for it and found it on the same site: https://www.ecatholic2000.com/lagrange/interior1/interior.shtml
Hope it is helpful to you. Excerpts also were on http://www.christianperfection.info/
earlier but I think that site is down of late.
The Three Ways of the Spiritual Life on Our Lady's Warriors, https://www.ourladyswarriors.org/saints/3ways.htm
is also useful.
It is by the same author, saintly Dominican Fr. Reginald Garrigou Lagrange, someone Archbishop Lefebvre often praised.
I'll just post a brief excerpt from the first and last chapter of the second work; the Interior Life is the One Thing Necessary. God bless.
"CHAPTER 1 : THE LIFE OF GRACE AND THE IMPORTANCE OF THE FIRST CONVERSION
THE interior life is for all the one thing necessary. It ought to be constantly developing in our souls; more so than what we call our intellectual life, more so than our scientific, artistic or literary life. The interior life is lived in the depths of the soul; it is the life of the whole man, not merely of one or other of his faculties. And our intellectual life would gain immeasurably by appreciating this; it would receive an inestimable advantage if, instead of attempting to supplant the spiritual life, it recognized its necessity and importance, and welcomed its beneficial influence -- the influence of the theological virtues and the gifts of the Holy Ghost. How deeply important our subject is may be seen in the very words we have used: Intellectuality and Spirituality. And it is important to us not only as individuals, but also in our social relations, for it is evident that we can exert no real or profound influence upon our fellow-men unless we live a truly interior life ourselves.
The necessity of the interior life.
The pressing need of devoting ourselves to the consideration of the one thing necessary is especially manifest in these days of general chaos and unrest, when so many men and nations, neglecting their true destiny, give themselves up entirely to acquiring earthly possessions, failing to realize how inferior these are to the everlasting riches of the spirit.
And yet St. Augustine's saying is so clearly true, that ' material goods, unlike those of the spirit, cannot belong wholly and simultaneously to more than one person. '  The same house, the same land, cannot belong completely to several people at once, nor the same territory to several nations. And herein lies the reason of that unhappy conflict of interests which arises from the feverish quest of these earthly possessions.
On the other hand, as St. Augustine often reminds us, the same spiritual treasure can belong in its entirety to all men, and at the same time to each, without any disturbance of peace between them. Indeed, the more there are to enjoy them in common the more completely do we possess them. The same truth, the same virtue, the same God, can belong to us all in like manner, and yet none of us embarrasses his fellow-possessors. Such are the inexhaustible riches of the spirit that they can be the property of all and yet satisfy the desires of each. Indeed, only then do we possess a truth completely when we teach it to others, when we make others share our contemplation; only then do we truly love a virtue when we wish others to love it also; only then do we wholly love God when we desire to make Him loved by all. Give money away, or spend it, and it is no longer yours. But give God to others, and you possess Him more fully for yourself. We may go even further and say that, if we desired only one soul to be deprived of Him, if we excluded only one soul -- even the soul of one who persecutes and calumniates us -- from our own love, then God Himself would be lost to us.
This truth, so simple and yet so sublime, gives rise to an illuminating principle: it is that whereas material goods, the more they are sought for their own sake, tend to cause disunion among men, spiritual goods unite men more closely in proportion as they are more greatly loved. This principle helps us to appreciate how necessary is the interior life; and, incidentally, it virtually contains the solution of the social question and of the economic crisis which afflicts the world to-day. The Gospel puts it very simply: ' Seek ye first the kingdom of God and his justice, and all these things shall be added unto you. ' If the world to-day is on its death-bed, it is because it has lost sight of a fundamental truth which for every Christian is elementary.
The profoundest truths of all, and the most vital, are in fact those elementary verities which, through long meditation and deep thought, have become the norm of our lives; those truths, in other words, which are the object of our habitual contemplation.
God is now showing men what a great mistake they make when they try to do without Him, when they regard earthly enjoyment as their highest good ... The present state of the world is called a crisis. But in fact it is more than a crisis; it is a condition of affairs which, if men only had eyes to see, ought to be revealing, it ought to show men that they have sought their last end where it is not to be found, in earthly enjoyment -- instead of God. They are seeking happiness in an abundance of material possessions which are incapable of giving it; possessions which sow discord among those that seek them, and a greater discord according as they are sought with greater avidity.
Do what you will with these material goods: share them out equally, make them the common property of all. It will be no remedy for the evil; for, so long as earthly possessions retain their nature and man retains the nature which is his, he will never find his happiness in them. The remedy is this, and this only: to consider the one thing necessary, and to ask God to give us saints who live only on this thought, saints who will give the world the spirit that it needs. God has always sent us saints in troubled times. We need them especially to-day ...
It suffices to read in the Dark Night  the description of those faults of beginners which render the purgation of the senses necessary. Here are, not faults opposed to the sort of psychological purity of which our author speaks, but faults which are contrary to the moral purity of the sensibility and of the will. They are, in fact, as St. John of the Cross tells us, the seven capital sins translated into the order of the spiritual life, such as spiritual greed, spiritual sloth, spiritual pride.
The same remark may be made of the faults  of proficients which render necessary the passive purgation of the spirit; they are ' stains of the old man which still remain in the spirit, like a rust which will disappear only under the action of an intense fire. ' These proficients, says St. John of the Cross, are really subject to natural affections; they have moments of roughness, of impatience; there is still in them a secret spiritual pride, and an egoism which causes some of them to make use of spiritual goods in a manner not sufficiently detached, and so they are led into the path of illusions. In a word, the depth of the soul is lacking, not only in psychological purity, but in the moral purity that is required. Tauler has spoken in the same sense, solicitous especially to purify the depth of the soul of all self-love, of all more or less conscious egoism. Hence it is our opinion that the passive purgations are necessary for this profound moral purity. But these purgations are of the mystical order. They do not always appear under so definitely contemplative a form as that described by St. John of the Cross; but in the lives of the saints, even of the most active among them, like a Vincent de Paul, the chapters which treat of their interior sufferings prove that they all have a common basis, which none has described better than St. John of the Cross.
A final and very important concession has been made to us in connection with the famous passage of the Living Flame, ST. II, 23:
' It behooves us to note why it is that there are so few that attain to this lofty stale. It must be known that this is not because God is pleased that there should be few raised to this high spiritual state-on the contrary it would please Him if all were so raised -- but rather because He finds few vessels in whom He can perform so high and lofty a work. For, when He proves them in small things and finds them weak and sees that they at once flee from labor, and desire not to submit to the least discomfort or mollification... He finds that they are not strong enough to bear the favor which He was granting them when He began to purge them, and goes no farther with their purification.... '" May we all strive to be saints, as God wishes for each of us.