Definately the Trevor Wye fisrt book on tone. This is excellent! My tone doubled in quality in just 3 days (*note - I practice about 3-4 hours a day so it may take more than three days)
I have Essential Elements 1 and 2, Play Flute Today! (volume 1 I think), and the elementary Rubank book.
My band director wanted everyone to get a new book over the summer, anything we wanted. Based on the comments so far, I'm guessing a Trevor Wye book would be advisable?
They are excellent books, and quickly becoming a staple for many teachers. If you do get it, the first five come together in the Omnibus edition, which is a great deal, and has so much information and music. I use this, and I can highly recommend it.