Thank you. I do highly recommend reading Understanding Comics someday. Like you, I'd already been reading comics for years when the book came out. It's far more than just a primer on navigating the comics page and comprehending the format. You'll find that you've inherently understood many of the ways that the comics form "works its magic," but McCloud verbalizes it so well it adds a new clarity and an idea how to explain these concepts to non comic readers.
I think I'm on my third copy of the book from all the times I've lent it out and just let a couple people keep the copy!