I have heard different things about the short plot overview paragraph in the query letter. Is it better to tell the ending of the book in the query letter or to leave the reader of the query letter hanging about the end of the story, as in the blurb on the back of book covers?
You are confusing the query letter with a synopses. You don't need to reveal all in the query letter, but you do in a synopses. A query letter requires the hook, and some introduction to who and what an agent is going to read on your enclosed pages. When we are reading your query letter and pages, we aren't concerned much about plot. That comes later, after you've established you can indeed write your way out of a paper bag...or in this case, an envelope.