Heres a LED light I picked up a couple weeks ago at a local hardware store for $4.
It has a swivel light that can be angled to any position you wish to direct the light. It can be clamped to a shelf and directed at another. Overall lenght of the light is less then 3 inch. Its very bright and it uses the small flat batteries I forget what thier called. Try the automotive section at Walmart I think I may have seen them there or try Home Depot, Lowes, lighting stores or your local hardware store. Thier a great alternative if you don't want to wire-up your shelving unit.
PROS
No heat emitted what so ever
No wiring needed
Doesn't add to the electrical bill (batt operated)
Postion it anywhere you want
If you use more then one light you can control which light(s) you want on or off
CONS
Manually have to turn each light on or off, not a terrible thing but some may feel its a pain in the arse
No dimming