I use a shoe polish brush for mine, never used on shoes ofcourse, works great and can be found easily.

As to what dust is made of, that all depends on where you live. If you live in a very dry climate (like say Amarillo, TX where I grew up) the majority of the dust is made of sand and fine dirt particles, and if you want a dust free bookcase you are going to do it everyday. I remember having to dust as a kid and never liked it cause by the time you got finished dusting everything in the house the stuff you started with was dusty again.