Squarespace "book lists" are built from product tags. Now, when a book goes out of stock, Bookhead removes the title from those lists so your curated lists (staff picks, new arrivals, etc.) only show what's actually on hand, then adds it back when you restock. (Technically, Bookhead removes the tag from the product -- taking it out of the book list on your website -- and sets the product as unlisted so it's not viewable in the storefront. It will still exist as a product in Squarespace and in the Bookhead book list).
FYI: any lists you've built directly in Squarespace aren't managed by Bookhead, so those we leave alone.