I've been asking around what is the difference between them but all I got from them are mere example or they would just shrugged off but the worst or funniest reaction I got is they would just stare at me like I suddenly got two heads (hahaha...). In the end, I figured it out myself by analyzing how was it use in a sentence.
My analysis is that "A/An" is use when you're generalizing while "The" is use when you're referring to something specific.
The first example is referring to specific book.
Let's read the book.
The second example is referring to any kind of book.
Let's read a book.