Heard this term over the weekend on SlashFilm, a movie news site.
A "knockbuster" is a cheap rip-off product that capitalizes on a highly anticipated product coming out soon. These are easier to spot in the film world, as you can see direct-to-DVD movies hitting stores just before a huge movie hits the theaters. Example: Why go see IndianaJones IV in the theater when you can rent the fantastic Allan Quartermain and the Temple of Skulls? - the latter of which is on DVD and available now. (Hooray!) Other terrible examples include Snakes on a Train and AVH: Alien vs. Hunter.
In the book world, the latest mystery having to do with Da Vinci or the Knights Templar would be a good example of a "knockbuster."