How many books in the Bible?

Well that's easy. As we all remember, the Old Testament has 45, the New Testament has 27, and the entire Bible has 72. We all remember that because Sister Mary Mnemonic taught us that the individual digits in each of those numbers adds to 9:
4 + 5 = 9
2 + 7 = 9
7 + 2 = 9

So today, my smart-alecky daughter says "Wrong. There are 46 books in the Old Testament. It says so right here in the Catholic Study Bible:"

Although it is bound under one cover and bears a single title, the Bible is not a single, unified book. It is, in fact a collection of some seventy-three [What!] different works by different authors, . . . . Most of the Old Testament (forty-six [Again, What!] books) was written in Hebrew, but parts of . . .

73?!
46?!
What's going on here?

We can reject immediately the suggestion that we might be misremembering Sister Mary Mnemonic's instruction. And we are encouraged that The Catholic Encyclopedia agrees with us (and with Sister):

The most explicit definition of the Catholic Canon [and it is the Catholic canon that we're talking about, of course] is that given by the Council of Trent, Session IV, 1546. For the Old Testament its catalogue reads as follows:

The five books of Moses (Genesis, Exodus, Leviticus, Numbers, Deuteronomy), Josue, Judges, Ruth, the four books of Kings, two of Paralipomenon, the first and second of Esdras (which latter is called Nehemias), Tobias, Judith, Esther, Job, the Davidic Psalter (in number one hundred and fifty Psalms), Proverbs, Ecclesiastes, the Canticle of Canticles, Wisdom, Ecclesiasticus, Isaias, Jeremias, with Baruch, Ezechiel, Daniel, the twelve minor Prophets (Osee, Joel, Amos, Abdias, Jonas, Micheas, Nahum, Habacuc, Sophonias, Aggeus, Zacharias, Malachias), two books of Machabees, the first and second.

I leave it as an exercise for the reader to confirm that this list does, in fact, add to 45.
That's 45 . . . as in 45!

So where does the Catholic Study Bible (and the Jerusalem Bible, and the Good News Bible -- which an old friend used to call the Reggie Jackson Bible) get the idea that there are 46 books in the OT?

The books listed by Trent are all in the Catholic Study Bible -- after allowing for a few name changes and a lot of spelling differences -- but (obviously) one book in the Catholic Study Bible is not included in Trent's catalogue. And that book is . . . ta dah! . . . the Book of Lamentations!

How did Trent miss Lamentations? Near as I can figure it didn't. I think Lamentations, which follows immediately after the Book of Jeremiah, used to be included as kind of an epilogue to the Jeremiah:

The Gk and Lat tradition . . . [attaches Lamentations] to the name of Jeremiah, and often preceded by the notice, "And it came to pass, after Israel was taken captive and Jerusalem was laid waste, that Jeremiah sat weeping and lamented with this lamentation over Jerusalem and said . . . "

The New Jerome Biblical Commentary, p. 280

So it appears sometime between 1546 and 2010, some know-it-all decided to split Lamentations away from Jeremiah and into a book of its own.

I leave this too as an exercise for the reader: Identify the know-it-all and show why his idea was a bad one.

Footnote 1:
Can we mention Jeremiah, even in passing, without mentioning Hopkins' GREAT poem? Well, um, no:

THOU art indeed just, Lord, if I contend
With thee; but, sir, so what I plead is just.
Why do sinners’ ways prosper? and why must
Disappointment all I endeavour end?
Wert thou my enemy, O thou my friend,	5
How wouldst thou worse, I wonder, than thou dost
Defeat, thwart me? Oh, the sots and thralls of lust
Do in spare hours more thrive than I that spend,
Sir, life upon thy cause. See, banks and brakes
Now leavèd how thick! lacèd they are again	10
With fretty chervil, look, and fresh wind shakes
Them; birds build—but not I build; no, but strain,
Time’s eunuch, and not breed one work that wakes.
Mine, O thou lord of life, send my roots rain.

Source

Footnote 2:
It would be an easy (dare I say trivial?) matter to derive a mnemonic for 46 OT books, 27 NT books, and 73 books total. For example, those numbers use all the digits from 1 to 9 except 1, 5, 8, and 9. 1589, as you must remember, is the year that the fourth Dalai Lama was born.