I remember speaking with someone about a book written by a late Roman scholar who understood that Rome was collapsing and that a large body of knowledge would be lost. Therefore, he wrote a book that tried to contain the entire body of Roman knowledge. I'm trying to find the name of the scholar and the name of that book.
That fits quite well with the Etymologies by Saint Isidore of Seville, a bishop from the VI-VII century Spain. The Etymologies are as close to an encyclopedia as you can get.
Another author, closer in time to the fall of Rome, was Saint Augustine, who wrote a colossal number of works, none of them encyclopedic, though.