The Intellectual Aspect of Life.

Intellectual development isn't as complicated as most people seem to think (especially considering that most books you'll find are 80% garbage).
It get even simpler once you've developed the right skills.
Just get some concentration under your belt (that's the hard part), then work on your memory (I think the Qabalah offers one of the best mnemonic/peg systems), then work on your information processing abilities (ie. photoreading and image streaming). Once you've got all these things highly developed you'll be able to understand everything and be able to do it in a quarter of the time.
As for the topics to consider I think everything is good to learn. There are, however, a few that stand apart from the rest.
First you got the obvious ones Math, Classics, and Science. Just dive in and have at er. Don't stop until you're perfectly satisfied.
Other than that anything to do with language is really valuable, and psychology and philosophy can be a lot of fun.
The main point should always be to get an understanding of something not knowledge.
It's so much less important to be able to memorize a bunch a facts than it is to be able to understand the larger concepts. Thankfully Photoreading and the Qabalah help immensely with this.