The Elements of Typographic Style →
I wish to see this book become the Typographers' Bible. — Hermann Zapf
For me this book is a masterpiece on typography and one of the few I'm really happy to own as a printed edition. If you're a least a tiny bit interested in typography this should be on your shelf already.
In case you're wondering: I was talking quite a lot about typography in the last weeks so I simply had to put this somewhere.