I recently picked up some books I've been promising myself to read for years, one of which is Patsy Rodenburg's extraordinary The Actor Speaks.
While much of Rodenburg's book is the building blocks of actually making and manipulating sounds, there is a LOT (and I mean lot) of the philosophy of acting and LIFE mixed in. Best of all, coming from the British tradition of theatrical training (of which I'm very proud to be a part myself - not a day goes by when I don't thank the RNCM in one way or another for what they gave me) voice and body technique is not skipped in favour of emotive expression, but the why's and wherefore of how one turns that physical technique into text and, ultimately, heightened emotion are explored ... which is precisely the journey we take as singers.
One of the best books I've read in years - I can't believe I waited this long to buy it. I can see it's going to become a constant companion, since so much of it easily "crosses the floor" to be entirely relevant to the operatic process (both physically and theatrically).