There isn't really that much to Bon-Ihrno's home. Wander the outside for a little bit and you'll find some Toadstools in the northeast and some Mandrake in the northwest.
When you first enter the house itself the first thing you'll see is Bon-Ihrno sitting at the table. Talk to him, paying close attention to what he says. He'll tell you many thing you need to know to destroy the Keep of Rentar-Ihrno.
In the southeast corner is a door leading to a storage room where Bon-Ihrno has left some supplies for you.
North of the supply room is Bon-Ihrno's bedchamber, and north of that is a room you can rest in. I've heard that if you sleep in this room Bon-Ihrno will die. I've never found out myself, since I'm too tender-hearted and never actually tried it.
Just to the west of the chamber prepared for you is another small storage area with a few pretty useless items.