Most effective Room Treatment for slap echo, etc

I am unfamiliar with room treatments products to eliminate slap echo, and to improve focus and imaging. I have no space in my living room to place two upright book cases stuffed with books to dampen down the room. Therefore, need advice and suggestions for a simple and reasonably priced room treatment solution. A member recommended Auralex. I rent my space,so wall or ceiling panels must have easily removable adhesive and not strip paint or stucco off the walls. Thank you, Jim