It's not the delay between each book, but rather, the time taken for EACH book to be sent.
Right now I'm sending one. 5 minutes running time have elapsed according to Calibre. One book took 7 minutes to send.
Running time for this book, 10m 50s...
The book size is 2048KB (2 megs), so it's not a small file, I s'pose.
I might reconfigure Calibre to send it via my ISP's email service, rather than via Calibre. I'll just have to add my personal address to the accepted addresses list in Amazon (Manage My Kindle).