Seems I recall some little kerfluffle about Jerome and having to strongarm him into including certain books.
Jerome’s issue was with the translation from his Greek and Hebrew sources for the NT and the OT, respectively. Some of the books, he could find no Hebrew source, and like a good translator, wants access to the oldest and best documents.
What we know today, and something of which Jerome was unaware is that some of the books were never written in Hebrew (having been written after 500 BC and the Exile), they were included in the Septuagint in Greek, and there exists no Hebrew originals.
So when Jerome was translating these books, he understandably was concerned, and expressed his concern to Pope Damasus. Damasus and the Church decided to include all the books that were present in the Septuagint, without reservations.
We’re not quite sure what books Jerome translated from. We don’t have them today, and we don’t possess his manuscripts. The oldest hebrew text that we possess is the Masoretic text, which dates to around 900 AD, and there are differences between Jerome’s texts and the Masoretic texts.
This was one of the more exciting discoveries in the Dead Sea Scrolls. There were some differences between them and the Masoretic texts, which confirmed Jerome’s translation. There were also some differences between both of them, suggesting that the source of the Dead Sea Scrolls was not the same as Jerome’s (400 years divide the two).
So we don’t know what he was using, but we do know that it was pretty accurate, and in some ways more accurate than the Masoretic texts. Which is also to be expected as there is 500 years between the two.
What would be nice is to find these texts, one of which was Origen’s Hexapla, which is now lost. It had an interlinear translation between 6 different variants of the Septuagint, and dates back even earlier than Jerome.