Your next steps will be your shining moment, along with CSS it will put the final nail on your coff.., I meant the finishing looks to your awesome website!
This final.. look would make your website move like never before, from changing texts dynamically to showing/hiding content using a single button within the same page! It allows for Personalisation, the first step towards making special tailored apps using this scripting language!
Going back to the book, if you wish, means you get to learn how to properly syntax your commands, select elements by Class,ID,Tags and even HTML Objects. jQuery, a product of JS is a library that would take your multiple lines of JS and compress it into commonly used methods to ease your job.
JS could be inline, within <script></script> tags in the head or end of body and, just like css, on seperate files for each function. Just make sure to add <script src="js/add-content.js"></script> at the end of body to allow for the script to run after the page is loaded.
Now make sure to use inspect element, get your favourite sites loaded and explore the different functionalities.
Now get exploring, for real.