Henadic Theologian
Legend
It seems that the non-core books sell the best at the pre-order stage. From older conversations (primarily about GW products) the bulk of purchases are within the first month to three months of a books, with a sharp drop-off after that. These days I imagine a good bulk of these books have already been on pre-order, probably for a couple months. Don't know how that might affect sources like the USA Today booklist.
It is interesting how huge of a price drop the Bigby book has though - I'm seeing it on Amazon at 44% off (and Phandelver is already 38% off; Planescape is 30% off; Book of Many Things which is to be released next doesn't appear to have a discount at all yet). I wouldn't expect a book that's selling like hotcakes to have such a huge discount so soon after it's release.
I think Planescape will be an exception to that because of what happened with Spelljammer. So many folks are like I'll wait and see if this is a train wreck like Spelljammer, because they felt burned when they pre-ordered Spelljammer.
I only bought Planescape on preorder because October 23 is my Birthday and I was curious if the Wizards Digital/Physical Bundle will be worth it (never bought any D&D Beyond stuff before).