It's just ridiculous. For the entire three-year production of Star Wars: Episode III--Revenge of the Sith, George Lucas and Lucasfilm went to Department of Defense-esque lengths to keep the plot of Episode III top secret. Then, almost a month before the film's release, they cashed in on the secrecy they'd created. Books have become a massive part of the Star Wars empire: Check the Star Wars section at Powell's, and be stunned by its size, thoroughness, and variety. The New York Times Bestseller List has been similarly overwhelmed--whenever a new Star Wars-branded book comes out, it makes more money than a year's worth of poetry sales. (I just made that statistic up, but I'm pretty sure its true.)
No less than five major Episode III books have come out in the weeks prior to the film's release: The Art of Star Wars: Episode III and The Making of Star Wars: Revenge of the Sith (both by J.W. Rinzler), DK Books' glossy Visual Dictionary to Star Wars: Episode III, Dark Horse Comics' graphic novel adaptation by Miles Lane and Doug Wheatley, and Matthew Stover's requisite novelization. (This list doesn't even count James Luceno's hilariously titled prose prequel, Star Wars: Labyrinth of Evil.) True, Dark Horse's comic boasts cool art and a few new plot points. And the Art of coffee table book is crammed with truly stunning images. But that's not the point.
The point is that eager, anxious fans were put in a shitty place when the books came out so early. They could try to resist buying them, or they could cave, spoiling the film's plot before they could even see it. Lucasfilm knew that once Star Wars fans see the film of Episode III, their curiosity will be sated--and their propensity to buy tie-ins will plummet. Hence the pre-Episode III book releases--plot spoilers be damned.
Cashing in on fans' anticipation makes perfect business sense. But what about respecting their privilege and their right to see the film, for the first time, without knowing every line of dialogue and exactly how that opening space battle is going to look? That's clearly a notion as pass as buying poetry instead of, say, Labyrinth of Evil.