I don't know the book, but there are different pkg_add(1) tools on different BSDs, which have different options.
What you're looking for on OpenBSD is the -E option. Review the man page.
Code:
$ pkg_info -E $(which convert)
/usr/local/bin/convert: ImageMagick-6.9.9.30
ImageMagick-6.9.9.30 image processing tools
$