Your link is to the current release of the book NOT the draft version as explained on the site... "Note: this document is the current release of HtDP/2e. It is

Your link is to the current release of the book NOT the draft version as explained on the site... "Note: this document is the current release of HtDP/2e. It is updated in conjunction with PLT software releases, roughly in sync with semester spans. It is thus well-suited for courses. In contrast, the current draft changes on a frequent basis; it should be consulted when people discover problems and/or errors in this document. If such flaws exist in both documents, please report them to the first author."