Any tips for decorating? + search in level editor?

[Adny]

It feels weird to call them "decorations", these are abandoned castles and ruined mansions, not a birthday party : /

Anyways, I'm assuming OP wants to know a little bit more about detailing than bookshelves, yes? Each individual mapping set (castle/dungeon/cellar/mansion) has specific things you can add to it to make it look more detailed. For castle/mansion, furniture can be added as a sort of "base" in which to add things from the ornament section on.

If you're going for that broken down/ruined look, think "cause and effect". If your ceiling breaks, there will be a significant amount of debris on the floor, perhaps a few pipes leaking water.

Also for castle/dungeon/cellar base, think "structure", this isn't exactly highly detailed, but logical and very noticeable. If you can add a few support beams to your ceilings/walls, it will keep them looking "busy" instead of boring and empty.

MOST IMPORTANTLY: Study the Frictional Games maps in TDD. If you study what looks good with each set, it will do you wonders I can assure you no one on these forums are experienced with HPL2 as the people who created the engine! Eventually you will become a pro at creating a perfect, highly detailed and spooky atmosphere! I could write a novel on this subject and how to do it, but I believe it can't really be taught, its something you have to get a feel for by yourself. Good luck!