I'd say the best way to learn more about vim if you are already a heavy vim user is to join both the vim-dev and vim users mailing list in google groups. People post questions in the vim user mailing list every day and I've learned many new things just by reading the answer that other people gave.
"if you master the techniques in this book, you’ll never need another text editor. In 120 Vim recipes, you’ll quickly learn the editor’s core functionality"
If you're a heavy vim user then you probably already know the editor's core functionality.
"and tackle your trickiest editing and writing tasks"
IMO once you get past knowing the core stuff on an editor, the next frontier is scripting. If you would consider yourself advanced in vim, I would suggest starting to learn VimL