Winn - Melodies for developing tone

I recommend the following:

Robert Winn - Developing Tone through Interpretation

'This method book is a collection of melodies to stimulate and encourage flute players to understand the neccessity of listening to and playing the music of other instruments and styles, and perhaps on the way help them to discover some of the elements which are indispensable when developing one's interpretive skills through the medium of the flute sound.'

This fantastic and popular book is aimed at students above Grade 5 standard.  It's a great book to use when focusing on tone, and allows you to explore different tonal colours through short musical extracts.