+1 vote for Practical Typography. It's simple and straightforward, and it's oriented toward the average person rather than professional designers. And since it's available in full on the web, you don't have to pay anything to start reading it.

Practical Typography is excellent. Excellent. I think he makes some hyperbolic statements at times, but Butterick as a whole is very accessible.

The only annoyance was ignoring Butterick's plugs for his personal typefaces.

I was very happy to buy a copy of his typography for lawyers book for a friend as recompense for this resource.

I was actually okay with the plugs for his own typefaces. Since the book was free, I thought it was fair that he suggest people buy his typefaces to pay for the free product especially since many will be taking his advice to create products that are intended to make money.

But yes, an absolutely accessible and excellent resource. I've been sending to links to various pages and chapters to my friends in the hopes that they'll choose better typefaces after reading.