I think I'm getting dumber. What are some good books to read?

I wasn’t pushed very hard to read as a child, and I’m paying the price. I think my mind is slipping from monotony. I’d like to try and start a book per month program. What are some good books that I can read that will be challenging, interesting, and enjoyable? I’m looking for books that are easier to read than Howard Zinn’s “People’s History of the US” and more difficult than Harry Potter.

Subjects I like: philosophy, technology, nature, science fiction, fiction, fantasy, etc. But I’m open to anything.

Any suggestions?