Things to remember:

• This proof had images at 1200dpi and 600dpi, well over the optimal 300dpi submission resolution. It is indeterminable. So might as well send in at 300dpi. That way you can send in more than 48 pages.
• Use less saturated colors. Pure blacks are going to drench the pages. Lower the levels a bit.
• Do not expect detailed heavily painted artworks to print well. Only use sharp contrasty flat colors.
• Try to think about your layout in advance and what types of fancy splash images you want to print. That way the ink is evenly distributed across the page. This is a limitation of using Lulu.com, but other than that, the print quality is awesome.