I've defeated the Overseer. Now what?

Question

I've reached the Overseer's Chamber, lit the area up, defeated its inhabitant and ate his eye. This opened no new areas to explore, however. Now what?

Defeating him didn't complete any quests I had, while I still have quests open such as "The Shimmering Deep" or "A Cure for Corruption" that apparently don't find an answer here and now.

Answer 1

The Shimmering Deep is an optional sidequest that involves finding a piece of ember, so it would not be completed by finishing the boss, and the ember may very well be further down below Torchlight.

As far as "A Cure For Corruption" goes, you need to find the Overseer's book after defeating him to complete the quest. I found some details on the Runic forums below on the location of the book.

When you defeat the Overseer, a bridge gets formed to the bottom-right of the area where you fought him. Cross this bridge and you'll find the book you need, and the stairs further down.

Taken from: http://forums.runicgames.com/viewtopic.php?f=5&t=2242