yeah, i agree with you it is totally different album.
Good Girl is more pop and RnB, if you are listening this album you just wanna have fun. lyrics sound funny, beats are so cool and everything ...
A Letter To Myself is more matured, that is true, but it is 2009 album, two years since her debut, so i am not surprised it sounds different.
In 2007 (when i first listened this album), Sherman was just "next debut singer", and she brought sexy female RnB style into new era of canto pop, so that is the reason why i like this album.
but maybe you will fall in love with GG later
it was just your first listen
So my fav. albums by Sherman:
no. 1. A Letter To Myslef
no. 2. Good Girl
no. 3. Castle
no. 4. Thunder Party