Hi gioe1e,
A 680 is a fantastic "baseline" score. Considering that you're not planning to take the GMAT until November, you're in a great position to potentially score at a really high level.
While most Test Takers cannot "peak" by taking a book-heavy study approach, you might be able to, and in a shorter period of time than most other Test Takers.
As a way to gauge your progress, here's what I suggest: study as you see fit for the next month or so (you can use whichever books most interest you), then take a FULL-LENGTH CAT (including the Essay and IR sections). Once you have those scores, you should report back here. Those numbers will give us a sense of how you're progressing and whether you need to adapt your approach.
It's possible (and even quite likely), that you could take the GMAT far earlier than you planned.
GMAT assassins aren't born, they're made,
Rich