Controlling external electronics

Hi Folks, It's getting cold and i'm looking for a project to keep me busy in the winter. My question is, how difficult is it, or where should i start reading when it comes integrating an arduino with pre-existing hardware (ie: tape deck, children's toys). I've thumbed through "Hacking electronics" by Simon Monk, but it's not exactly what i'm looking for. I'm more interested in "hijacking" existing electronics and controlling them this way.

Any information regarding this would be really appreciated.

The easiest way to get started is to replace switches in the electronics at hand by a Arduino controlled relay...