A really solid philosophy book is Moral Issues in Global Perspective, editor: Christine Koggel. This is the best collection of ethics essays out there. It is very interesting and covers some topics that would be relevant for pre-meds. Another important aspect is that it also exposes you to numerous writing styles since every article is written by a different person. The people range from Kant to Nussbaum making it very diverse. You can get it for very cheap used from amazon (there's even a person selling it for 97 cents.
A good Aristotle book would be Nicomachean Ethics. If it matters at all, this is coming from a philosophy student.
There is also philosophical acid trip that is Zhuangzi.
In general, I'd say most philosophy isn't tough to understand, people just get confused by all the terms philosophers make up and how they use different definitions for particular words.
. I would like to start reading those forms of articles to get comfortable with them.
