In the web development world and even web design, you can’t memorize everything. That’s just silly and impossible. 10 years ago most developers had a bookcase full of books for reference. Many still do that today but it’s also very easy to use online resources.
Google has a developer resources section as do all the various programming languages, the W3C, and so on. When you have a quick question you can typically Google it and come up with an answer, tutorial, and code very quickly to help you with your project. If not, a message on a forum may lead to an answer.
This is good and all for reference material but there are still things that I find having a book to teach the concepts and procedures is irreplaceable.
You could take a college class easy and spend all your time learning about the core of WordPress. I’m busy and don’t have time for classes nor have the real need for a teacher. So instead, I do what they used to do long ago and pickup a book! There’s something about going to the bookstore and thumbing through some books on a topic. Finding what you need and reading a few pages before making the purchase. Then heading back to your computer and read and develop!