TimW wrote

My study is being redecorated after the need to dig under the
floor to replace the incoming water main. I am refitting the fitted
bookcases across one wall and I have a wire in place to add some
lighting. I want lights behind the cornice of the bookcases shining
onto the ceiling for a nice gentle dffused effect.

In the few years since I last looked everything has gone LED
and I have no idea what kind of fitting I need. An LED cord?

Those work very well in that situation and is very easy to do.

A batten? or battens?

A LED cord works much better in that situation.

In the olden days you might have used fluorescent tubes, but not now.

Still viable now but no point anymore instead of a LED cord.

The bookcase is all of one wall, 2.5m wide and the cornice is about
300mm below the ceiling. I have some foil tape to make it all reflective.

What sort of light is needed in this situation?

:LED cord would work well.