There is a typo in Code listing 4.2 SRC_URI has "mirror//" which should be "mirror://" Rather simple fix. Reproducible: Always
As I've said before, docs in /proj/ belong to those individual projects. Reassigning; not the GDP's turf (you want the Java team for that).
Fixed: -SRC_URI="mirror//gentoo/${P}.tar.gz" +SRC_URI="mirror://gentoo/${P}.tar.gz"