How much information about the plot of your book would you recommend revealing in a query letter? Up to the climax? How the story concludes?
I think the best advice I’ve heard on query letters is to make your book description sound like the sort of blurb you’d read on the back of a published book. Before you get started, read the blurbs for some of your favorite books, and see how they do it. Give a little set-up, introduce the main character and his/her main conflict, what’s at stake, maybe a complication or two, and that’s it. You definitely want to hold off on telling the ending–you want the agent or editor to be eager to read the story and find out what that ending is!
For the query for GIVE UP THE GHOST, which you can check out here, with my agent’s comments, I covered (in a very general way) the events of about 2/3 of the book. But you can see I didn’t give away how Cass resolved her problems. And you can definitely cover less, as long as what you do tell is gripping and shows how your story is unique.
For example queries that work and don’t work, and thoughts on why they do or don’t, I highly recommend checking out Query Shark and Evil Editor, as well as Miss Snark’s Crapometer archives (start at the link and work backwards). And read those book blurbs!