• A beginner's guide: https://www.janefriedman.com/a-beginners-guide-to-amazon-pre-orders/
• BookBub: https://insights.bookbub.com/promote-preorder-books-drive-real-results/
• More from BookBub: https://insights.bookbub.com/should-you-make-a-book-available-for-preorder/
• And even more: https://insights.bookbub.com/preorder-strategy-build-books-platform/
• Some authors use crowdfunding to pre-sell their book, which essentially acts as a pre-order mechanism outside of Amazon.