If you're like me, as a small publisher, you're learning some sides of the business in bits and pieces. In my case, I had the writing skill, but no graphics or printing knowledge. And sometimes, if you're learning things in bits and pieces, you'll get the wrong understanding of some of the jargon used by the other guys involved in the process. In fact, the better they are, the more likely that they'll assume you know something you don't.
In my case, one of these misunderstandings was about the term "bleed," when applied to printing a cover. I managed to go at least three years with a mistaken understanding of what this meant. As far as I understood it, "bleed" was just a layout artist's weird way of saying that the artwork was supposed to go to the edges of the page. Almost all the time, you can tell an artist that the graphic is "full bleed," and they'll just say, "okay," and you'll get what you need.
However, it becomes an issue if you want two pages to lie next to each other and match up at the borders (this is how my mistake got revealed). Because here's what "bleed" really means:
When the artist produces something with full bleed, it means the graphic is actually bigger than the space you want to fill. It's slightly larger, in the case of cover art, than 8.5 x 11. Part of the artwork is designed to spill off the edge of the page, and this is called the "bleed."
What's that all about? Isn't this whole process digital? Well. if you think about it, there's one part of the process that isn't entirely digital. Think about how, when you print a document, your home printer shakes a bit as it moves blank paper into place and prints on it. The presses used by book printers do the same thing. They vibrate, they shake, and they jiggle while printing. Thus, the page of paper can't be guaranteed to be EXACTLY in the right place. The bleed is a bit of excess picture that will get printed if the page is off center by a fraction or two of an inch. If the graphic is precisely and exactly the size of a sheet of paper, you risk having a white line along one side if the paper is off center in any direction.
So, if you're printing on lulu or somewhere similar, and you've ween white lines along one edge or another of the cover, it's because you didn't have any bleed outside the margins of the page.
This little note will probably only help a few people who make the same mistake I did, but since it happened to me, it might have happened to other publishers with no graphic/layout experience.