Both Uses of

discreet

in

Glory in Death

She kept her voice down and a carefully pleasant expression on her face. "Lieutenant, must you do this now, here and now?"
"Mrs. Whitney, I'll be as discreet as I possibly can."
Chpt 6 *

discreet = trustworthy with secrets and/or inconspicuous or unobtrusive

One entire area was a treasure trove for those of a vegetarian persuasion, with another, at a discreet distance, decked out for carnivores.
Chpt 20

Definitions:

(1) (discreet as in: discreet--not showy or gossipy)
trustworthy with secrets or subtle (not attracting attention)

This sense of discreet sometimes indicates that something is small (so as not to attract attention) as in "discreet earrings."
(2) (discreet as in: discreet--not foolish)
sensible and careful; possibly having good manners

This sense of discreet is more commonly seen in classic literature than in modern writing. There is may also refer to having the judgment to exercise good manners.