Glass-fronted bookcases.

The best way to do it is to take down each shelf's books, dust the shelf, then dust the books individually with a fluffy duster and put them back.

In between these thorough cleanings, I just knock the dust off the front of the shelves.

I've got one of the older Dysons, and vacuum both the library wall and the DVD shelves on a regular basis, with the soft brush attachment stuck on to the hose, no wand- saves me HOURS, which means I can take my time with the OLD books, which I usually do by hand, with one of those microfiber cloths...

'dusting' even with a wool duster, just makes me sneeze - vacuuming actually removes the dust

I do the same thing as chinacat - I use the round brush attachment of my vacuum cleaner for dusting all kinds of things.