I am going to attach our bookshelf to the wall and am wondering what type of anchor I should use. The wall consists of very porous plaster. About 5 cm in, there's something hard that I don't know what it is. I have been recommended a heavy-duty anchor for lightweight concrete but want more opinions before I use it. If it doesn't work, I would have a gigantic hole in the wall. Admittedly, behind the bookshelf, but it still feels unnecessary.
How old is the house? If you have plastered walls indoors with that thickness, it could be reed walls and then wooden planks behind. Try screwing a long thin screw straight in and see if it holds. If the screw doesn't hold, it is likely concrete. But it's strange to have such thick plaster on a concrete wall indoors.
Drill a small hole and see what's behind it, as Fnurpern wrote, if you're lucky it's wooden planks, then you can just screw straight in. Otherwise, it's probably brick, then you'll need to drill with a hammer drill or impact drill. If you have patience, it may also work with a screwdriver if it's just a few holes.