The Book of Daniel - Chapter 13 (Susanna)

The Book of Susanna, absent from the Masoretic text, but present in the Greek Septuagint (which was generally considered an accurate rendering of the original Hebrew) tells the story of two Hebrew Elders who lust after a pious, moral, married woman Susanna.

We see their tactics of intimidation and deception when she doesn't comply with what they want, but rather places her Trust in God (much like the 3 Young Men in the Furnace as we saw in the Prayer of Azariah).

What are the lessons we need to learn here, and why was this book hidden from the original Masoretic (Hebrew) text?