https://wappalyzer.com/download - one of my favorite tools - will give you a little more insight into what smartsheet uses - they're homepage at least, is a LAMP stack with Drupal.
Smartsheet would be a tall order for your first or 5th project. Get a cheap server somewhere - start with PHP, Rails or Node and build stuff. Learn how to communicate from the frontend to the backend - make mistakes, rinse repeat.
I haven't bought or read a book in years; once you get the basic concepts down, searching google / stackoverflow for how to do something will come much easier.