A number of their books were only found in Ge'ez, the classical Ethiopian language, similar to Latin and Classical Greek today.
I have always wondered about the Ethiopian canon.
The most interesting books included in it which are not found elsewhere are 1 Enoch and Jubilees, however, 1 Enoch does survive in fragments among the Dead Sea Scrolls, and also in Latin and Koine Greek fragments, thus attesting to its authenticity; also St. Jude appears to quote from it in his epistle. I believe Jubilees is likewise attested to a similiar extent, except not quoted as far as I know in the New Testament. There are a few other different books, and the Broader Canon appends the Didascalia, a book of church order similiar to the Didache, to the end of their Bible, theoretically in the New Testament, but the prevailing canon is the Narrow Canon, which only has extra books in the Old Testament and otherwise follows the 27 book Athanasian Canon for the New Testament.
Here is a link to Wikipedia’s article on the Ethiopian canon, which is quite thorough (and which one can readily validate through other sources, like the Blackwell Guide to Eastern Christianity):
Orthodox Tewahedo biblical canon - Wikipedia
Now concerning the language, Ge’ez is the liturgical language of the Ethiopian and Eritrean Tewahedo Orthodox Churches; they are one of two Oriental Orthodox churches that only uses a liturgical language, the other being the Armenian Apostolic Church, which exclusively uses Classical Armenian. The Coptic Orthodox Church, the Syriac Orthodox Church, the Malankara Orthodox Syrian Church (also known as the Indian Orthodox Church, despite the fact the Syriac Orthodox Church is also in India and about the same size) and the Malankara Independent Syrian Church (which uses Oriental Orthodox liturgy but is in full communion with the Mar Thoma Syrian Church, which is a member of the Anglican Communion) all make use of Coptic and Syriac as liturgical languages, but since most Syriac Orthodox do not natively speak Syriac, and Coptic is extinct outside of the liturgical context, although both the Syriac Orthodox Church and especially the Coptic Orthodox Church are doing a really good job teaching their youth these languages as second languages, the church services are conducted in a mix of Coptic or Syriac with vernacular languages.
Arabic is ubiquitous in the Coptic and Syriac churches, due to geography, and likewise Malayalam, the Syriac-influenced language spoken in the Malabar Coast / Malankara State region of India, which interestingly enough unlike Hindu is not an Indo-Iranian language (like English, French or Greek), nor is it a Semitic language like Hebrew, Syriac Aramaic, Arabic, Ge’ez, or Amharic (the prevailing vernacular language among the Ethiopians, not to be confused with Aramaic; its basically a descendant of Ge’ez just as Modern English is descended from Middle English and Old English). Coptic by the way is an Afro-Asiatic language.
In the diaspora, the language of the country of migration tends to be heavily used in these churches, and indeed the Syriac and Coptic churches in the US and Europe want to phase out Arabic so that their people there will basically worship in the local vernacular, mainly English, German, Swedish and Dutch, and in the traditional liturgical language (although in the case of the Syriac Orthodox Church, there are still a small number of native Aramaic speakers using dialects similar to Classical Syriac like Turoyo; there are also native Aramaic speakers in the Antiochian Orthodox Church, in a town in Syria that was occupied by Al Qaeda in 2014, which held the nuns at the local convent hostage for a few months and ransacked the churches, although amazingly they were not anywhere near as messed up as ISIS, which was genocidal. Also, speaking of churches persecuted by ISIS, in addition to all the churches I have mentioned in the Middle East and Africa, there is also the Assyrian Church of the East, which worships in Classical Syriac like the Syriac Orthodox, but with a different accent, and where 70% of the laity still speak Aramaic, specifically Assyrian Eastern Neo Aramaic, making them the largest population of Aramaic speakers in the world.