Performing GET and POST requests using Ajax http://www.javascriptkit.com/dhtmltutors/ajaxgetpost.shtml
Submit Form Using Ajax, PHP and jQuery | FormGet https://www.formget.com/submit-form-using-ajax-php-and-jquery/
Using FormData Objects – Web APIs | MDN https://developer.mozilla.org/en-US/docs/Web/API/FormData/Using_FormData_Objects
Submitting AJAX Forms with JQuery ― Scotch https://scotch.io/tutorials/submitting-ajax-forms-with-jquery
Submit Form Using Ajax, PHP and jQuery | FormGet https://www.formget.com/submit-form-using-ajax-php-and-jquery/
Use wp_localize_script, It Is Awesome | Pippins Plugins http://pippinsplugins.com/use-wp_localize_script-it-is-awesome/
Building a WordPress Plugin to Create an AJAX Contact Form – SitePoint http://www.sitepoint.com/building-a-wordpress-plugin-to-create-an-ajax-contact-form/
How to make jQuery ajax (JSON) requests in WordPress | Pittsburgh Web Design & Development– Andrew M Peters http://www.andrewmpeters.com/blog/how-to-make-jquery-ajax-json-requests-in-wordpress/