The other two major tiers of book-marketing writing are your tagline and your summary.
1. Taglines should invite, inspire, or both. You’ve heard a thousand taglines in your life. Most of them business related, like Nike’s “Just do it.” But, for example, Serenity‘s “Can’t Stop The Signal” is a perfect tagline: it’s relevant to the movie’s plot as well as a rallying cry to the short-lived-but-wicked-sweet Firefly series’s fans. Star Trek‘s “To boldly go where no man has gone before” invites you to join the journey with the characters. A snippet of dialogue, a phrase that insinuates more than the sum of its parts (see the Scott Pilgrim quote below), or becomes significant after the reading is what you’re after.
2. Don’t be trite or generic. You’re going to see this show up a lot as a rule, because there’s a lot of writing out there that does what the last popular cliche’-storm sci-fi or fantasy deal did before. If your tagline is “An epic fantasy/sci-fi adventure”: spare us. Scott Pilgrim did better than that by both repeating the same word three times and misusing it with “An epic epic of epicness.” Ditto for any tagline that regurgitates a tired cliche: “Only Main Character could wield the sword against the Dark Lord …” Your tagline should attempt to advertise that which is unique or interesting about your story’s world, characters, or hook.
3. Don’t spell it out. Taglines are meant to grab someone’s attention or interest, or at least make them ask a question. They are not meant to answer the question. Nike’s “Just do it” is not “Just jog” or “Just play basketball”. It doesn’t spell out what you’re supposed to do; it inspires you to fill in the blank. Similarly, the phrase is not “To boldly go into deep space, make love to green women, and discover planets.” “Can’t Stop The Signal” at least teases the viewer with “What’s the signal? Why can’t it be stopped? Is someone trying to stop it?” A tagline should entice the so-inclined, not the stupid people who need stuff spelled out for them.
Run your tagline by your Ideal Reader, preferably one who has read the work and can give you an honest answer about its suckage level. Once you’ve done that, run the tagline by some Ideal Reader folks who haven’t read it and ask them if it interests them. Preferably the Internet doesn’t count, but if that’s your only go to, knock yourself out.