http://peterkirby.com/the-quotable-josephus.html
The works of Josephus did not survive only in complete editions of his books. Between the fact that Josephus was (and remains) eminently quotable, particularly for his relevance to the times of the New Testament, and the fact that his works were voluminous, there was often occasion for excerpts to be made from his texts.
Excerpta (or florilegia) were a common form of (subliterary) writing activity, from antiquity, through the medieval period, and into modernity. Extracts were made for personal use, in preparation for one’s own composition, for straight quotation or adaptation in other works, and even for publication in their own right. As Goldberg’s helpful web page on the New Testament Parallels to the Works of Josephus illustrates, Josephus is particularly well-suited for this kind of anthologizing.
Sometimes the text of Josephus is mediated by the quotations of Josephus from Eusebius, and this may be particularly applicable in the Latin and Syriac versions of Eusebius, when the translated quotations in Eusebius may sometimes have been the only source for the Antiquities of Josephus available to the writer in that language. There is, for example, this example of a so-called Letter from Pilate to Herod: “These letters occur in a Syriac manuscript in the British Museum, dated in the sixth or seventh century. There is a Greek copy in a Paris museum.” At the end of it are appended three quotations, apparently attributed to Justus of Tiberias, the emperor Tiberius, and Josephus. The quote from Josephus, regarding Herod Agrippa and his death, receives the following note: “This extract from Josephus is abridged from the account of Eusebius in his History of the Christian Church.”
Alice Whealey notes that this also appears to be the case for this quotation of Josephus, Ant. 17.168-170, by way of the Syriac translation of Eusebius, HE 1.8.5-7, found in Michael’s chronicle, itself in Syriac...
The quotation found here, like the previous abridgment in the Letter from Pilate to Herod, regards the death of a Herod found in the New Testament, this time the death of Herod the Great.
These extracts are not useful merely on their own, as possible witnesses to the original state of the text. Knowledge of these extracts, in specific and in general, also help us to figure out what options there are for the likely source of quotations from Josephus found in other texts. It is possible, for example, that the quote from the chronicler or the apocryphon above is not only mediated from Josephus by Eusebius but also mediated from Eusebius by way of a collection of excerpts. Such texts were convenient, economical, and ubiquitous, particularly in use in preparing other historical works. Often they were made by the author for the purpose of their own use, but they also were also often shared, published, and used by other authors who quoted an author such as Josephus from extracts.