The Complete Guide to High-Fire Glazes and Clay and Glazes for the Potter by Daniel Rhodes.
At present I'm immersed in A Potter's Book by Bernard Leach. Written in 1940, the book is beyond classic. It's perhaps the most important ceramics text in the Western world because of Leach's incomparable knowledge of Eastern pottery, culture and aesthetics.
Leach was born in Hong Kong to British citizens. As a young man he studied art in Japan under the tutelage of the great master Kenzan VI, and worked alongside future legends Kawai Kanjiro, Tomimoto Kenkichi and Shoji Hamada. He returned to England with Hamada and established his own studio, and is credited with reviving the ceramics craft movement in the face of industrialization.
Personally, the book is reinforcing many of the things I've learned to date and giving me a good perspective on not just "how" but "why" things are done in ceramics. It's very interesting to read a book written 70 years ago that's absolutely relevant today. And Leach has a very subtle sense of humor that's doled out very sparingly throughout the 260+ pages. Leach is not so subtle with his criticism, though, of which the book is absolutely filled. Conversely, it's also filled with rather unabashed praise for pottery from the Chinese Tang and Song dynasties.