I
heartily recommend
The Waste Lands. It's the best book of the entire series, in my opinion (well...the final book is darned good too, but that's because...well, it's the final book!) So I'm quite sure that if you enjoyed the first two books, you'll definitely enjoy the third one. I hope you do, anyway, it is a very good read. It starts off very dramatically and maintains the momentum throughout. Go for it, I says!
And I also certainly recommend reading some of King's other novels. For the something more eerie and unsettling you could try
The Shining,
IT,
'Salem's Lot,
Pet Semetary... But often King's villains aren't of the supernatural variety, so if you're perhaps more interested in that you could perhaps try
Misery,
Rose Madder,
Dolores Claiborne etc. I know there are some people who aren't so keen on his style (personally I really like it) but if there's one thing that people usually agree on, it's that King creates some brilliant stories.