Many book publishers use Indigo digital presses which are arguably the best of that technology. However, due to it's screening, it can have issues with neutral colors (output a page of just a solid neutral, you'll see the effect of the screening whereby the neutrality moves throughout the page). That said, when I've tested output from Aperture, it's been pretty good but if you demand dead nuts neutral across the page, any such digital press is going to drive you a bit nuts.