As far as I'm aware Ibn Caspi is the first and only Rishon to address the question, as he wonders why the book of Exodus begins with a "vav":
לא נדע באמת *מי היה ראשון להפסיק הפרשיות גם לעשות מהתורה חמשה חלקים*, ונכון מאוד שהיה משה עצמו מפי השם ,...".
We will never know who was the first to divide the Parshiot, and who made the Torah into 5 parts. And it seems very true that Moses himself did so, instructed by God.Ibn Caspi doesn't really add to our knoweldge. However there are some Midrashim which seem to suggest that the division into 5 books wasn't always a given:
בר קפרא פתר קריא בתורה, בנתה ביתה זו תורה שנאמר ה' קנני ראשית דרכו, חצבה עמודיה שבעה אלו שבעה ספרי תורה, ולא חמשה הם? בר קפרא עביד מרישיה דוידבר...".
Bar Kapra explains the Pasuk in Mishli "Wisdom has built her house; she has set up[a] its seven pillars. " as referring to the SEVEN books of the Torah! (see the geomorah in שבת קטז עמ' א where an opinon of Rava is mentioned that "ויהי בנסע אהרון" is a book upon itself - Rashi explains that those who see the torah as seven books, have really divided Bamidbar into 3).
I have a dim memory of the Zohar stating there are 9 books, but I couldn't find a quote online. I was also somewhat surprised at the lack of sources on this issue, and I somewhat expected someone to have suggested that all the Torah used to be one book. I'm very interested if anyone has any more sources on this issue.