Finds the div with the most p tags, then adds CSS Columns to it. I use http://mrcoles.com/bookmarklet/ to convert it to a bookmarklet.

horz_reader.js

function callback() {
    (function ($) {
        var h = $(window).height() + 'px',
            w = '25em',
            g = '2em',
            r = '1px solid #eee',
            most = 0,
            content = $('body');
        if ($('#content').length == 1) {
          content = $('#content');
        } else {
          /* find div with most p tags */
          $('div > p').parent().each(function(i, elm) {
              var $elm = $(elm);
              var l = $elm.find('p').length;
              if (l > most){content = $elm; most=l}
          });
        }
        content.hide();
        content.css({'-webkit-column-width': w, '-webkit-column-gap': g, '-webkit-column-rule': r, '-moz-column-width': w, '-webkit-column-gap': g, '-webkit-column-rule': r, 'column-width': w, 'column-gap': g, 'column-rule': r, 'height': h, 'overflow-y': 'hidden'});
        content.fadeIn();
        $('body').append($('<style />').text('img{max-width:100% !important;height:auto !important}'));
    })(jQuery);
}
if (!jQuery) {
  var s = document.createElement("script");
  s.src = "https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js";
  if (s.addEventListener) {
      s.addEventListener("load", callback, false)
  } else if (s.readyState) {
      s.onreadystatechange = callback
  }
  document.body.appendChild(s);
} else {
  callback();
}