What book would you recommend for the principles of baking?

For all of the technical details and a pretty thorough understanding of the principle you really can't go past Rose Levy Beranbaum's Cake Bible or Bread Bible.

The Cake Bible by Rose Levy Beranbaum

The Bread Baker's Apprentice by Peter Reinhart. Also, I think Carol Fields' The Italian Baker just came out in a new revised edition.

King Arthur's is pretty darn good.

Bakewise by Shirley Corriher