Consulting the dead is clearly condemned in Deuteronomy 18:11. We also see Saul attempting to do this just before his death in 1 Samuel chapter 28.
As to the matter of soul sleep, I would look at texts like these:
The Bible teaches that there everyone who dies will be raised from the dead, either to everlasting bliss (heaven), or to everlasting torment (hell).
You may also wish to look at these lessons on the BSF Website: