[tikhonj]

Books are constrained as much by medium as readability. If you want the book printed cheaply, you're relegated to black text on an off-white background. Also, there's a big difference between text on a back-lit display and text on a page.

I think dark gray text is a good choice for text on computers.

[sliverstorm]

What the dark grey (it isn't really super dark) text actually achieves here is to allow your attention to be drawn to the pictures of book covers.

This, of course, makes the site prettier, because book covers are colorful.

Personally though, I'd rather just have a list of [book name - author(s)]