His Dark Materials trilogy - Philip Pullman
Superb, Practically anyone can read it, and it has some very interesting things to say on theology and region without ever becoming side tracked
The Day of The Triffids - John Wyndham
modern survival horror has nothing on this, it pretty much created the entire genre
Dracula - Bram Stoker
Most Victorian literature is too dry or too slow, but Dracula still reads really well today. Again, this started a genre, and still works, if not a full horror, which today seems obsessed by trivial matters such as gore
1984 - George Orwell
Anyone with an interest in politics should read this. It's shocking just how possible the world Orwell creates is
The Crucible - Arthur Miller
Hands down my favourite play. Really, Really can't recommend it enough
The Lorax - Dr Seuss
"Business is business! And business must grow, regardless of crummies in tummies, you know."
Honestly, just read it, I don't care if it was intended for younger audiences, it still can make anyone think a little. Plus the art is fantastic, perfectly capturing the mood. (No, the film doesn't count, it comes nowhere near to capturing the mood of the book)
And I really agree with YCHTT, Shakespeare is well worth a read, even if it's hard going sometimes