I have selected a bunch of notes with Ctrl-Click and Shift-Click, done what I needed to do, and now I want to work on the next bunch of notes.
but my old bunch of notes stays selected!
How do I deselect them?
 (Tried double-click, shift-click, click again, ctrl-click, alt-click, right-click OOPS! deletes note... I have looked in the Help, found nothing)

ctrl-shift + A to unselect all..
 
ctrl-click to unselect 1 notes or more..

With the Smart tool you can left-click any empty area in the PRV to deselect everything.

Yes, if you want to start working on a new selection, just right click to the left and above the first note in the new range and drag to highlight the additional required notes.
 
To use an Excel analogy, you would put your cursor in the top left cell of the required range prior to dragging

Wow. Yes, thanks. Here's what happened:
 
First I selected all notes in one bar and edited.
Then I ctrl-clicked on a note I wanted to de-select. This did not work.
 
But I notice there is a grey area at the top of the piano roll view.
It is dark grey over all my selected notes. Those can no longer be de-selected.
 
But if I click anywhere EXCEPT on a note, (or any of the techniques you guys suggested),
ONLY THEN does the dark gray area go away and everything works as normal again.
 
Had me fooled. Thanks again!