Check the Books
As you probably already know, Coda has an entire mode devoted to reference material. While you can just sit down and read it like a book, or search it for certain terms, it's much faster to look things up directly from your editor session.
Say you're working in PHP, and you want to use a function like str_replace() but you can never remember the number or order of the arguments.
If the word str_replace already appears in your code, simply hold down the command key and double-click on the word. Books will open, and take you right to the str_replace() page.
You can accomplish the same thing by selecting the word in the editor, and pressing command-' (apostrophe)
If you haven't typed str_replace yet, turn on the hints banner, by clicking the Hints button at the bottom of the editor. As you start to type str_replace, you'll notice that the Hints banner shows you matching PHP functions, until it finally narrows in on the one you want. That may be all the hint you need, or you can click on "More..." to go to the full books page on str_replace().
And of course, if you use auto-complete to complete the word str_replace, Coda will insert the function arguments for you, but they'll be selected, so you can just type over them with your real code.
(Note: These examples assume you are using the PHP-HTML syntax mode. But other syntax modes work similarly.)