I can see how Christian books could be useful; however, there are so many Christian books out there; Christian books may just be too broad. You might still find yourself having to explain and search for more specific adjectives.
Here are some additional suggestions:
- The 27 Books
- Primary Apostolic Writings
- Primary Christian Writings
Reading through the conversation in the comments to this answerthis answer, about Holy/divine writings, I am reminded of an old term, "Holy Writ," and wonder if the word writ might be employed:
- Apostolic Writ
- Christian Writ
Christian Writ strikes me as an exceptional over-all option, in that it:
- defines the collection specifically as what the Christians/Apostles wrote,
- is concise and natural enough to bear up under repetition in lengthy discourse,
(is even one syllable shorter than "New Testament.") - respectfully conveys that the Christians consider the collection divine/holy,
without implying whether one agrees or disagrees with their conviction. - writ does not seem to be used for other Greek/Christian writings
(such as the church Fathers, or contemporary books/documents).
Christian Scripture also puts ownership of the conviction on the Christian; however, it may not be distinct enough to represent only the books written by Christians, since the whole of their bible, both parts together, is held by Christians as Scripture.
A bit more eccentric:
- The Christian Rule of Faith
- The Christian Anthology
It might be helpful to refer to the collection by its components:
- Gospels, Acts, Letters and Revelation,
either list them, or refer to the particular part that is pertinent to conversation.
In reference specifically to the original Greek manuscripts, some of the above suggestions might be adapted by replacing writings with documents, manuscripts, parchments, or corpus.
A bit too obscure:
Early Christian writings
Greek Christian writings
Apostolic writings
Greek Canon
Christian Canon
Greek Canon-used to refer to collection of Greek art. One might think using earliest instead of just early would remedy this; however, earliest has connotations concerning (carbon) dating of the manuscripts and if you merely intend to imply foundational, then, primary would be the word of choice, as foundational would be a mouthful.
I don't intend to solve the matter with "an answer" but rather to present a compendium of vocabulary, terms, and adjectives adjectives; to consider the various implications, connotations, and limitations; to thus help equip/prepare those who endeavor to communicate respectfully and effectively about such matters in inner faith dialogue.