Once you've published a few books, your editor will probably contact you while you're still finishing a book to say, "I need a brief synopsis of the plot to give to sales." Encapsulating a whole plot in a few lines is a real challenge and it takes a lot of practice to do it effectively. One of the best ways I've learned how to do this (which is the same thing an agent/editor wants to see in a QUERY---take note, new writers!) is to make a visit to the library or bookstore and start to read the backs of books.
What the sales department often wants to see is the "hook" of the book. Short, snappy and filled with vague details that make you want to pick up the book to find out more. It's how they're going to sell it to the distributors and book buyers (who also want short and snappy---because that salesperson is only one of a dozen they might see in a day.)
But what should be included and what should be EXCLUDED? Here's what I've learned so far and I would love other authors, as well as readers, to share what you like to see in back cover copy:
1. The names of the primary protagonist(s.) At least first names. If a romance, give me BOTH names so I at least know what variety of book it is (M/F, F/F, M/M/F, etc.) If a UF or other genre, just the main person.
2. Tell me what's at stake in the broadest possible view. Will the world end if they fail? Will someone die? Will the ranch be lost and cut up for development? Will the person go bankrupt, wind up in jail or lose a child? Remember to be BROAD and use the worst case scenario. If the worst case isn't "worst enough" in your copy it's harder to sell it to someone who knows nothing about your people or world.
3. What does the person have to lose EMOTIONALLY? Will they have to compromise their values (or morals?) Will they have to disown someone, or be disowned? Will they wind up hated or feared or even fall in love? Remember--short and snappy.
4. Make sure your blurb doesn't require knowledge of any prior books. Sure, it's terrific if the sales people (or agent) already know your work, but really---you should give enough information for someone to pick it up as a FIRST book, even if it's the tenth in a series. Because people change jobs frequently in a city the size of NY. The salesperson assigned for your last book might not be the same person for this one, and might never have even heard of you.
5. Leave them with a question. Not necessarily with a punctuation mark, but with a "will they succeed? will they fail?" concept that is grabbing.
Here's what I came up with for the "rough copy" for Demon Song.
Celia Graves believed she'd severed the ties between herself and a demon who had plagued her but all the exorcism really did is delay the inevitable. For the unholy entity raised by the siren Eirene has found a new home---in a prison for the magically insane. A rift has opened between the dimensions, unseen since the destruction of Atlantis. It threatens to set loose all the demons of hell upon humanity. Celia has the tools to close the rift, if she can only discover how to use them in time. But to overcome the death curse which nearly guarantees her failure, she'll have to join forces with people she never wanted to trust again, along with those she fears getting closer to.
Obviously, the editor made some . . . well, edits. But a lot of it remained as I wrote it. This is only six sentences (even if a couple are compound), which fills the bill of "short" for an editor. Did I succeed or fail in implementing my own rules? Would you want to read more? Let me hear you!