This handlebars helper is for grunt-assemble 0.4.x. Newer versions of assemble make this easier and probably not needed at all.

include-helper.js

var extend = require('extend-shallow');
var fs = require('fs');

module.exports.register = function (handlebars, options) {
  // you should be able to set `cwd` on assemble options in 
  // your grunt config
  var opts = extend({cwd: ''}, options);

  handlebars.registerHelper('include', function (filename) {
    // to see what's on the context, do the following:
    console.log(this);

    var fp = path.join(opts.cwd, filename);
    return fs.readFileSync(fp, 'utf8');
  });
};