As to those OT books in contention, Luther in part, apparently relied upon Jerome's opinion of them, which most likely was influenced by those coming before himself.
That the Hebrew "canon" (the Hebrews had no such word as "canon" although they employed the concept) was limited to what is now found in "protestant" bibles, was not a new concept. Luther did not invent the point of view. His own view was similar to that which Jerome held. Don't let yourself be mistaken, for reason Jerome was directed to include what he himself referred to as Apocrypha.
Don't believe me? That would be ok, for I myself am no authority, and am reliant upon those whom can study the ancient languages.
I can however, provide link to those whom may, for one whom does seems to know a semi-ancient style of Latin. He provides link to the pages he is translating and excerpting from --- and is showing the source text itself as something of a "sidebar" footnote, with the work of the translated excerpt and commentary, thankfully provided in English. Otherwise I could not read much of any of it, at all.
For those reluctant to follow links, below is a portion of discussion, with excerpted translation apparently included, bracketed in quotation marks.
I don't know if it will be of any help, but here's a link to Jerome's Prologus Galeatus in Latin.
- First, “those books that are truly Scripture must be reverenced and adored” and should be distinguished from the non-canonical books “which the Hebrews include among the Apocrypha … Therefore here we have distinguished, and I have distinctly enumerated, first the canonical books, and afterwards the non-canonical ones: between which moreover stands as much distance as between certainty and doubt. For the canonical books were completed by the Holy Spirit who spoke them.”
- Second, “the non-canonical books, or the Apocrypha,” as Nicolas says, “are read for devotion and the information of our morals but nevertheless their authority for proving those things which come into doubt or contention and for the confirming of church doctrine is not reputed as proper … just as Jerome says in his prologue on Judith and upon the books of Solomon. But the canonical books have such great authority that whatever it contains is firmly true and cannot be shaken. For just as in philosophy truth is recognized through a reduction to first principles (principia) known per se: thus also the truth is recognized in the writings handed down by the holy doctors inasmuch as to those things that are held faithfully through a reduction to the canoncial scriptures which are held in divine revelation, which in no way can be considered false.”
- Third, a glowing reference to Jerome’s “helmeted prologue” (prologus galeatus) on the books of the Kings, at which point, Nicholas enumerates what is today the Protestant list of Old and New Testament books, then wearing Jerome’s helmet he goes to war against the canonicity of the Apocrypha enlisting Augustine, Rufinus, et al.