the spellbook is risky, because any lich with a full book can have ether lots of books or a really heavy one

in regards to traps, I recommend combining a teleport circle with a swinging scythe, swings, misses, bomes out of midair, strikes the wall, comes out the floor, disappears back into place

or maybe a huge rock that hits multiple circles, falling farther and doing more damage

or the rock hits a circle on the floor, and comes out a wall in the last room, smashing into the doorway, trapping them.