>perl -wMstrict -le
"my $book = qq{pg. 1 foo  pg. 2 bar baz  pg. 4 fee fie  pg. 5 foe \n}
          . qq{fum  pg. 6 hoo ha  pg. 9 deedle  pg. 10 \n}
          . qq{blah blah  pg. 14 noddle \n}
          ;
 print qq{[[$book]] \n};
 ;;
 my $pn = qr{ pg[.] \s+ }xms;
 $book =~
   s{ $pn (\d+) \K  (.*?)  (?= $pn (\d+))             }
    { my $m = missing($1, $3);  $m ? qq{$2$m  } : $2; }xmsge;
 print qq{(($book)) \n};
 ;;
 sub missing {
   my ($i, $j) = @_;
   ;;
   return if $j - $i < 2;
   ;;
   my ($ii, $jj) = ($i + 1, $j - 1);
   return $j - $i > 2
        ? qq{(pages $ii - $jj missing)}
        : qq{(page $ii missing)}
        ;
   }
"
[[pg. 1 foo  pg. 2 bar baz  pg. 4 fee fie  pg. 5 foe
fum  pg. 6 hoo ha  pg. 9 deedle  pg. 10
blah blah  pg. 14 noddle
]]

((pg. 1 foo  pg. 2 bar baz  (page 3 missing)  pg. 4 fee fie  pg. 5 foe
fum  pg. 6 hoo ha  (pages 7 - 8 missing)  pg. 9 deedle  pg. 10
blah blah  (pages 11 - 13 missing)  pg. 14 noddle
))