OK
How do I find a book to preorder?
Figured it out myself, so I'll share for anyone else doing this:
At the left of Amazon's search bar toggle from 'all departments' to 'books'. Click the magnifying glass to search, even though the search box is empty.
In the left menu (under 'show results for', 'New Releases' ) find and click 'coming soon'.
There's a 'sort by', but 'price high to low' didn't seem to work. I sorted by publication date and found some medical books coming out in Oct.
I ordered and it used up my GC. I then set up another purchase and it used my cc and didn't offer to use my GC. YAY! Thank you, yaakovl !
As an added point, if you've paid with a cc and it hasn't shipped yet, if you put in a GC to your account, you can go to your order and click to add the GC to the order.