Written by Aelfric of Eynsham (circa 955–1010) and first appearing around 1000 AD, the Excerptiones de arte grammatica anglice (Excerpts from a grammar textbook rendered into English) was the first Latin textbook written in English.
It is important because in it Aelfric coined the English language's first set of grammatical terms. For example, he translated the Latin terms declination and pronomen into the Old English declinung and naman speliend.
For additional information, see Aelfric's Grammatical Terminology by Edna Rees Williams in Publications of the Modern Language Association of America (PMLA), Volume 73, No. 5, Part 1 (Dec., 1958), pp. 453-462