The book isn’t out yet, so no promo code, but the whole thing is free online.
Thank you for your prompt response. I understand the item isn't available, but I clicked the "Add to Cart" button thinking it would either place me on a waitlist or ship once available. However, I was then prompted to enter a promo code, which caused the confusion.
Ah, weird! We can ask our publisher. This HN post was a surprise (Chis & I didn’t put it up) so we weren’t prepared.
I was able to pre-order the book just fine — it didn't prompt me for any promo codes.