When making gitweb.cgi (section 4.6), the prefix is set to /usr. If one builds Git from source like Scott suggested in the beginning of the book, /usr/bin/git doesn't exist and Gitweb won't display the project (check the Apache error log). The correct make line is:
make GITWEB_PROJECTROOT="/opt/git" prefix=/usr/local gitweb/gitweb.cgi
This is fixed in the second edition. Thanks!