This is mostly (I made some changes) copied from a post I made in a "what five books should everyone read" thread lost in the ENBoards meltdown, recovered through google. Technically they all have the potential to be "mind altering", though not necessarily radically so.
Atlas Shrugged (Ayn Rand)
It's not a bad story (though she could have used a good editor), but the main attraction is the ideas and philosophy laid out in the book. Even if you don't agree with all her opinions - and you probably won't - you can learn a lot by explaining to yourself why you don't agree. Rand is big on why.
The Uplift Series (David Brin)
It didn't strike me as all that brilliant when I first read it, but I find the ideas from these books regarding "uplift", intelligent life and interspecies cooperation coming back to me both in RL and while reading SF. (When I heard recently that African gorillas may be closer to extinction than previously believed my first though was "what lost potential that would be!".)
Dune (Frank Herbert)
A good book and it's ideas about prophesies in particular have stuck with me. It also has some interesting ideas about "power". (I'm not sure if I want to recommend the entire series, but as I recall I generally enjoyed the other books too.)
"The Culture books" (Iain M. Banks)
Ok, not really a series as such, but still related to each other. Again, interesting ideas and philosophical musings, involving "value/importance", freedom and AIs.
Flim Flam (James Randi) (Non-fiction)
Introduced me to the idea that maybe it's all bunk. Highly recommended.