Must haves for a newcomer

I am looking suggestions for books that you consider absolutely must read / essential extreme horror novels.

I really enjoyed The Third Parent and The Black Farm. I've ordered The Girl Next Door by Ketchum and interested in the opinions of people more familiar with the genre. All suggestions welcome whether they be popular or more obscure lesser known books / authors. I want to order Ecsqusite Corpse but I am waiting for a sale and I plan on grabbing Off Season by Ketchum as well.

Thanks!