The Bulletin of Abilene Christian College, June 1928, for the upcoming 1928-1929 academic year describes the campus facilities. It includes this description of the library:

Description of the library, Bulletin, Abilene Christian College, June 1928. p. 18. See https://texashistory.unt.edu/ark:/67531/metapth45915/m1/20/?q=bulletin%20abilene%20christian%20college%201928
About six months hence most of the library would be lost in a devastating fire. Plans were already underway to relocate to a new campus northeast of downtown Abilene, but the 1929 fire hastened the exit from the First Street campus. The library contained “nearly nine thousand volumes, about two thousand pamphlets and bulletins, and about fifty magazines and other periodicals on the essential fields of study and activities…” The Bulletin hails “two distinctive features” of the collection: 1) the “unusually large Bible department” and 2) the “careful selection,” further stating “Many volumes are denied place on the shelves because [they are] not standard, not moral, or not true to scholarship and constructive Christianity. Like the heart, a library is as valuable for what it keeps out as for what it has within.”
A collection of that size was reasonably adequate to support a “First Class” (see p. 17) four-year senior college curriculum. To my knowledge no specific detail survives which outlined the criteria for inclusion, or exclusion, of books from the ACC library. Therefore what I offer here is only a broad and suggestive first attempt. I will be pleased to learn of–and i will keep looking for– details which might color and inform my hypothesis.
I suppose the needs of the curricular offering were a major factor in collection development. At least a major practical factor guiding the selection and acquisition. At the same time and in a deeper way the stated purpose of the school undergirds a collection development policy such as the one outlined above. The curriculum functioned as a basis upon which to offer credible and recognized four-year ars baccalaureus degrees. And the library collection, as all libraries do, either served that end and facilitated that work to greater or lesser degrees. But secular course offerings, along with the intellectual and moral development they represent, served a greater purpose in the mind of those who operated the school. And the function of the library was at the conceptual core of the whole educational enterprise on North First Street, Abilene, Texas. Compare the statement above in the context of the paragraph below, ‘Purpose of Abilene Christian College’:

Purpose of Abilene Christian College, Bulletin, Abilene Christian College, June 1928. p. 17. See: https://texashistory.unt.edu/ark:/67531/metapth45915/m1/19/?q=bulletin%20abilene%20christian%20college%201928
By 1932 the 5000 books lost in the fire were replaced. With a few additions the collection grew 10, 147 books. “By special purpose in 1929,” notes the 1933 Bulletin, “a group of very interesting old books was added to the Rare-Books collection which now contains some volumes dating from as early as 1522. It has a collection of Bibles in seventeen languages.” (Bulletin, 1933, pp. 7-8). Margaret Bishop was librarian at the time. She graduated from ACC in 1924 (BA) and from Vanderbilt University in 1927 (MA), and later studied in the summer term at the Drexel Institute of Library Science in Philadelphia.
The library staff at the time, in ways consistent with the general academic outlook of the school at the time, was attuned to the currency of higher education and library science. They were not uninformed. Apparently among the take-aways from Drexel that Margaret brought back with her Abilene was an awareness of the value and utility of rare books in an academic library. She also reclassified the entire collection when she replaced the fire-damaged books. Appearances suggest Margaret ushered in a tangible commitment to modern library science, upgraded the collection and the way it was viewed and used by the school, in significant and enduring ways. In fact, by 1931 she was offering formal credit-bearing instruction in library science. She and her administration were wholly committed to the implications of operating an institution of higher education the aim of which was “the glory of God, through the Lord Jesus Christ and the ennobling of mankind.” This commitment, in the most fundamental way, informed and shaped their work and they were not afraid to guard the library collection like one would guard their heart.