Let me start out by saying I don't necessarily agree with everything he says, but I find his writing interesting. I have only read the books I mentioned once, and it has been a while ago for some of them. Sometimes I got the "Huh, are you sure?" when I read his books, but many times he made me stop and think. The Devil and Miss Prym is currently my favorite, it portraits the struggle between good and evil, light and darkness within the human being.
I don't know much about alchemy, but in my understanding alchemists were scientists. They used the natural elements; earth, air, fire and water in search for a deeper understanding of Nature (and possibly of God's Creation).
The way I read it, the boy in The Alchemist searches for his treasure, which he stubbornly believes is of material kind, but gradually discovers is more spiritual - the meaning of his life and the search for god. So he believes that alchemy will be the means to achieve it, but it turns out there are many ways to find one's treasure - but they all go through the heart rather than the mind.
Anyway, I was very inspired of his ideas of following your dreams and let the dreams live to inspire you in life...
[If you want to discuss it further but can't post here, because it will violate the religion rule, feel free to PM me
]