The Elements of Typographic Style by Robert Bringhurst
Quite possibly my favourite book ever. If judging the number of times I’ve read it is any indication.
I bought the first edition in 1992, immediately realizing it as the authoritative book it was. That it was beautifully designed, and written with the sort of grace a poet is capable of, just made it more appealing. (Bringhurst is a remarkable man, and this exemplary treatise on the subject is just one of his many achievements.)
I read that book cover to cover, over and over. The section on the Golden Section was worth the price alone.
I’d bought more recent editions as gifts for friends over the years, but never treated myself. While I was in Swipe in Toronto recently, I grabbed the fourth edition. I may not be as involved with graphic design and typography as I once was, but given how much I’ve enjoyed and gained from this book over the years, a second copy 25 years on was worth it.
Anyone who works with type to any degree, needs to own this book, and put the teachings in to practice.