See this legally free ebook:

  • The Blessed Trinity, by Rev. C.F. [Charles Francis] Blount, S.J. [1855-1931] (London: Catholic Truth Society, 1925). May be read online, and available in pdf, mobi, epub and other formats at Internet Archive.

See also the ebooks indexed on the page God and Christ, under the sub-list I.B. De Deo Trino; and the ebooks in the posts under the category God and with the word ‘Trinity‘.

On the Feast of the Most Blessed Trinity in the Latin Church. Kyrie eleison! Christe eleison! Kyrie eleison! For other texts and ebooks on various subjects, you may access the List of Free eBooks (Arranged by Title), the List of Free eBooks (Grouped by Subject), the List of Worth-It Catholic Books & eBooks, and the main page of the Catholic eBooks Project.

“I saw the Lord sitting upon a throne, high and elevated: and his train filled the temple. Upon it stood the Seraphims… And they cried one to another, and said: Holy, holy, holy, the Lord God of hosts, all the earth is full of his glory… And I heard the voice of the Lord, saying: Whom shall I send? and who shall go for us? And I said: Lo, here am I, send me.” (Isaias vi, 1-3, 8)

“Going, therefore, teach ye all nations: baptizing them in the name of the Father, and of the Son, and of the Holy Ghost” (Mt. xviii, 19)