You will get many detailed responses because it depends on your framing. Attic storage is done all the time without damaging the house.
I have seen people nail down sections of plywood onto the joists (a full sheet is too big to get up there). This adds strength laterally and does distribute some of the vertical load. Light boxes should not be an issue. A mountain of books is not good anyway because the heat will kill the books. Of course, you NEVER put plywood down over a wire that would be sandwiched between the plywood and the top of the joist!
However, a picture of your attic would help to determine the framing design. Do you live in snow country? What do you plan on storing?
Hope we can help you!