Philip K Dick

I've read A Scanner Darkly, Do Androids Dream of Electric Sheep, and Lies Inc.

Lies was a mind fuck.  It wasn't bad, but the majority of the novel consisted of the protagonist's experiences on LSD. 

What should I read next?  I've heard VALIS is more of a challenge than Dick's other novels.  What's it about?  What should I read up on to get a good understanding of what Dick is trying to say in VALIS?

I also hear Ubik is good.

Discuss Philip K. Dick.  Do it.

VALIS is a glorious book.  The first time I read it, it fucked with my head so much  You don't have to read up on anything to understand it.  You can quickwiki any of the various references to religions and philosophers that you may not already be familiar with.  (Some of them are fictional.)  If you're going to read it, though, you need to be prepared to read it as part of a series, and set aside the time to read the related novels.

The Divine Invasion
VALIS
Radio Free Albemuth
The Transmigration of Timothy Archer.

Those will keep you busy for a while.  Good luck.