As I was searching for reviews of different private pilot study kits (Sporty’s, Jeppesen, ASA, King, and others), I bumped into this:

Free Online Private Pilot School.

What a find: lots and lots of information available for free. It’s divided into categories (Airplane, Airmanship, Weather, Navigation) and then into topics, each topic containing multiple articles. By definition, I’m not an expert (otherwise I wouldn’t be looking), but it appears to cover all of the required material. I’m still going to purchase a course eventually (whatever the flight school uses), when my financial situation allows me to start going into actual training, but for now, this is definitely a good read to prepare myself.