I found the following book really useful which I found in the local library -

Set your voice free : how to get the singing or speaking voice you want - Roger Love

I found it useful in trying to smooth out the notes I could sing across the range of my voice and it has also definitely increased the notes I could sing at the higher end of my voice. One of the things he points out is that the loudness should NOT increase as you go up through the range, and he aims to get a smooth sound throughout.

I have only been singing for a few years having spent most of the first 50 odd years being scared of doing it. When I started singing I tended to sing in the lower end of my range and thought that the upper end sounded 'funny' - very light and weak and not like me.

I reckon that the exercises in the book have been really useful but whether it is a good teaching aid I don't know but it has been helpful to me (and a friend who I lent it to).

When I started singing I reckon that my range went from something like a lowish D to a comfortable Bb and a strained C or D 2 octaves up. I can now sing up to a G# or an A almost an octave above what I used to do though I rarely sing anything above an F# or G. I'm not sure if that means that my natural range has changed or whether I have just found my natural range!