the ‘nymph’ Carmentis.
Even William Smith’s authoritative three-volume Dictionary of Greek and Roman Biography and Mythology has little to add. The entry is reproduced (below, right) from Vol. 1. p.589. Rome’s old Porta Carmentalis is shown (upper register, below left). The map was published in the article ‘Rome’ in Smith’s Dictionary of Greek and Roman Geography Vol.2 Both texts are available through the internet archive.
Isidore seems to have relied here chiefly on Plutarch’s life of Romulus in his Parallel Lives of the Noble Greeks and Romans – though Virgil referred to Evander (Aeneid viii.338) and credited him with having brought the ‘letters and the arts of civilization’ to the early Romans. The second belief was general and is repeated by Livy and by Tacitus.
Plutarch was a Greek, but lived while Greek was “the language of literary and polite society at Rome.” I know of no complete Latin version before they begin emerging in Europe from the late fourteenth century; a majority of our remaining manuscripts in Latin or vernacular languages belong to the late fifteenth- to late sixteenth centuries.
Plutarch was not inclined to credit a woman with introducing letters…
Carmentis (also called Carmenta) had two feast days in ancient Rome; one on January 11th and the other on January 15th. They are called, together, the Carmentalia. For interest, I link to the western Christian calendar’s saints for those days: JAN 11th. JAN 15th.)
Another source which was available for later medieval Europe was the brief paragraph in Fabulae attributed to Hyginus. Printed copies of this text, and copies of translations in the vernacular presumably disseminated knowledge of Carmentis to the wider public. Isidore would not have described the Fabulae as a product of ‘liberal’ or polished letters; it presents as the veriest ‘common letters’ – as a school-boy’s ‘crib’ written by a scarcely-literate person and presumably by a youth.
Modern writers are accustomed to treat the Fabulae as the work of an eminent Roman, Gaius Julius Hyginus, despite its positively dreadful Latin and numerous elementary errors. One writer famously said that its author seems “adulescentem imperitum, semidoctum, stultum” – “an ignorant youth, half-educated and stupid.”
Here, for example, is a literal translation of entry 277 which mentions Carmentis (this paragraph isn’t included in the Latin transcription offered by the Latin Library online):
“The Greek (sic.) letters. Mercury is said to have brought to Egypt, and from Egypt Cadmus took them to Greece. Cadmus in exile from Arcadia, took them to Italy, and his mother Carmenta changed them to Latin to the number of 15. Apollo on the lyre added the rest.” 
- A site called ‘E-codices’ includes an excellent guide to digitised manuscripts,[HERE]. A fifteenth-century Latin translation of Plutarch’s Lives is among its listings, and the text (incomplete) is written in an easy, clear, late Latin humanist hand, on paper. (Fribourg/Freiburg: Bibliothèque cantonale et universitaire/Kantons- und Universitätsbibliothek, Cap. Rés. 527).
- Table of Contents to Plutarch’s Lives offered by the University of Adelaide [HERE].
- Manuscript copies of Plutarch’s Lives, by date and holding library available at Tertullian.org – ‘Some manuscript traditions of the Greek classics’ [HERE].
- The Introduction to the Loeb edition (1914) of Plutarch’s Lives, with the translator’s descriptive list of earliest extant manuscript copies is at http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Introduction*.html. Given the date of that excellent Introduction, you may need to check current locations for the manuscripts.
- Catalogue of Roman deities giving links to further information and pictures. [HERE]
- Porta Carmentalis. Stanford University has an ongoing project, reconstructing Rome as it was c.3rdC AD, from a marble ‘street directory’ being pieced together from remnants of the original and earlier drawings of it. A fascinating project: ‘Stanford Digital Forma Urbis Romae Project’. At the time of writing, however, the site provides no general ‘Search’.
- Cultic practices associated with Carmentis. [HERE]
- The Encyclopaedia of Ancient History has an excellent article: ‘Carmentis and Carmentalia’ but is accessible online only through a paywall at the Wiley Online Library.
- Margarita Philosophica was another encyclopaedic compendium. Written (in Latin) between 1489 and 1495 it first appeared in print in 1503. It was intended for young adults and university students. A blogpost at ‘BibliOdyssey’ [HERE] links to a number of digitised copies.