In my opinion, it is not necessary to use jQuery or XHR to accomplish this, since you can load the files as resources in the html header.
In this way load the files in the header of your HTML:
<script src="about.txt"></script>
<script src="main.txt"></script>
<script src="more.txt"></script>
In the body of these files declare the content as a variable:
about.txt
var about = '<div class=about> </div>';
main.txt
var main = '<div class=main> </div>';
more.txt
var more = '<div class=more> </div>';
And then when you load the page, the features you loaded in the form of text files will be available at variable level to use them, so you can manipulate them at will.
Example (after including the files):
console.log(main+about+more);
document.body.innerHTML = main+about+more;