I took basic design classes as part of my fine art degree, but haven't studied typography formally. I love finding resources like Butterick's Practical Typography available online. Although I do love the feel of a book in my hands, this website is an excellent example of an easy to read and well organized website. I would prefer longer pages, but they load quickly and it's filled with great examples.
This quote from the introduction explaining why typography matters really resonates with me.
Typography matters because it helps conserve the most valuable resource you have as a writer—reader attention.
Attention is the reader’s gift to you. That gift is precious. And finite. And should you fail to be a respectful steward of that gift—most commonly, by boring or exasperating your reader—it will be promptly revoked.
The book is only available online; a format that keeps it up-to-date.