From The Life and Revelations of St. Gertrude, Virgin and Abbess, of the Order of St. Benedict by a religious of the Order of Poor Clares.
Published by Burns, Lambert, & Oates, London, 1866. (available online here)
Part III, Chapter XXXIII: Of the value and importance of spiritual Communion, pages 218-220.
THIS holy spouse of Jesus Christ had usually an extreme and ardent desire to receive the Body of Christ, and it happened that once, when she prepared for Communion with more than ordinary devotion, she found herself so weak on Sunday night, that she feared she would not be able to communicate ; but, according to her usual custom, she consulted her Lord, to know what would be most pleasing to Him. He replied : ” Even as a spouse who was already satisfied with a variety of viands would prefer remaining near his bride to sitting at table with her, so would I prefer that you should deprive yourself of Communion through holy prudence, on this occasion, rather than approach it.”
” And how, my loving Lord, can You say that You are thus satiated 1 ” The Lord replied : ” By your moderation in speech, by your guard over your senses, by all your desires, by all your prayers, by all the good dispositions with which you have prepared to receive My adorable Body and Blood these are to Me as the most delicious food and refreshment.” When she came to Mass, though still in a state of extreme weakness, and had prepared for spiritual Communion, she heard the sound of a bell announcing the return of a priest who had gone to a village to give communion to a sick person.
” Life of my soul ! ” she exclaimed ; ” how gladly would I receive Thee spiritually, if I had time to prepare myself worthily ! ” ” The looks of My Divine mercy,” replied the Lord, ” will impart to you the necessary preparation ; ” and at the same time, it seemed to the Saint that the Lord cast a look upon her soul like a ray of sunlight, saying : “I will fix my eyes upon thee ” (Ps. xxxi.)
From these words she understood that the look of God produces three effects in our souls, similar to those that the sun produces in our bodies, and that the soul ought to prepare in three ways to receive it.
First, the glance of Divine mercy searches the soul, and purifies it from every stain, making it whiter than snow j and we obtain this favour by a humble acknowledgement of our defects.
Secondly, this look of mercy softens the soul, and prepares it to receive spiritual gifts, even as wax is softened by the heat of the sun, and becomes capable of receiving any impression ; and the soul acquires this by a pious intention.
Thirdly, the glance of Divine mercy on the soul makes it fruitful in the different flowers of virtue, even as the sun produces and ripens different sorts of fruit; and the third effect is obtained by a faithful confidence, which causes us to abandon ourselves entirely to God, confiding assuredly in the superabundance of His mercy, believing that all things will contribute to our eternal welfare, whether they appear favourable or adverse.
Then, as some of the community communicated at Mass, our Divine Lord appeared to give Himself to each with His own Hand, making the sign of the cross as the priest does; the Saint, marvelling at this, said to Him: “Lord, have not those who have received Thee in this Sacrament obtained greater grace than I, whom Thou hast gratuitously favoured with so many benefits? ”
” Who is esteemed most worthy,” replied our Lord, ” he who is adorned with pearls and precious stones, or he who has an immense treasure of pure gold hidden in his house?” making her understand by these words, that while he who communicates sacramentally receives without doubt immense grace, both spiritually and corporally, as the Church believes, still, he who abstains from receiving the Body of Christ through obedience and holy discretion, and purely for the glory of God, and who, being inflamed with Divine love, communicates spiritually, merits to receive a benediction like that given to the saint, and obtains from God more abundant fruit, although the order and secret of this conduct is entirely hidden from the eyes of men.
From The Blessed Sacrament by Father Frederick William Faber, D. D.
Tan Books and Publishers, 1978 (this edition originally published 1958 by The Peter Reilly Co., Philadelphia). (available online here)
Book IV: The Blessed Sacrament a Picture of Jesus, Chapter VII: The Life of the Church, pages 438-443.
Nothing can show [the Blessed Sacrament’s] power more wonderfully than that the very shadow of it should itself be one of the greatest powers on earth. I speak of Spiritual Communion, which is in truth the Communion of the angels. Nothing can show its power with Jesus more wonderfully than the innumerable times in which the desire of Communion has enabled the soul to receive the real Communion by some stupendous miracle, as has been already related of St. Catherine of Siena and other saints (…).
The Council of Trent  recommends it to the faithful; and St. Thomas  says, They are considered to be communicated spiritually and not sacramentally, who desire to receive the Sacrament; and they eat Christ spiritually under the species of this Sacrament; so that, as Scaramelli says , they do not only receive Jesus spiritually, but this very Sacrament spiritually. (…)
Joanna of the Cross  affirmed, that she was often visited in spiritual Communion with the same graces she received in sacramental, and, with a sigh, she exclaimed, O rare method of communicating, where neither leave of confessor nor superior is needed,but only Thine, O my God!
The Blessed Agatha of the Cross so pined with love of the Blessed Sacrament, that it is said  that she would have died if her confessor had not taught her the practice of spiritual communion, and then she used to make two hundred spiritual communions every day. (…)
St. Theresa  (…) speaks of spiritual Communion by the way, when the immediate subject before her is the disposition we ought to bring in order to receive our Lord worthily; and from this she is led to remark that these dispositions alone, even without the sacramental reception of our Lord, are productive of many graces to us. Her words are as follows: “Whenever, my daughters you hear Mass and do not communicate, you can make a spiritual Communion, which a practice of exceeding profit, and you can immediately afterwards recollect yourselves within yourselves, just as I advised you when you communicate sacramentally; for great is the love of our Lord which is in this way infused into the soul. For when we prepare ourselves to receive Him, He never fails to give Himself to us in many modes which we comprehend not.” (…)
It is said of St. Angela Merici that when she was forbidden daily Communion, she supplied by fervent spiritual communions in the Mass and often felt her heart as completely inundated by grace as if she had made a sacramental Communion, and she left as one of her legacies to her order and earnest recommendation of this devotion. 
Father Squaillante, of the Naples Oratory, in the life of Sister Mary of Santiago  (…), tells us that her love of the Blessed Sacrament was such that she had at last come to make a spiritual Communion at almost every breath she drew. (…)
In the life of Maria Scolastica Muratori, a Roman lady, by Father Gabrielli of the Bologna Oratory, we read that she tried to make a spiritual Communion every time she raised her eyes or drew her breath, so that, as she said, Were I to die suddenly, I should die as it were inhaling my God. 
 Sess. xxii. de Sac. Miss. c. vi. but especially Sess. xiii. c. viii.
 St. Thomas, Summa Theologica, p. 3 qu. 21 art. 1 and 2.
 Direct. Ascet. i. x. vii.
 Lohner, Bibliot. Predicat. Art. Commun.
 Barry, Année Saincte tome 3. p. 89.
 The Way of Perfection. chap. xxxv.
 Salvatori. Vita della Santa. lib. ii. cap. iii. p. 83.
 Life of Mary de Santiago (Vita di Suor Maria) by Father Niccolo Squillante, lib. ii. cap. iv. Available here on Google Books.
 Vita lib. ii. cap. viii. cap. xiii. lib. iii. cap. xi.