Not all languages have articles. English and Afrikaans are the same here. 'n Boek is a book, and die boek is the book. A ('n) is the indefinite article and THE (die) is the definite article.
'n and DIE: What is the difference?
"Hy is dors, gee vir hom 'n glas water asseblief."
"Hy is dors, gee vir hom die glas water asseblief."
Here, 'n glas water means any glass of water. He is thirsty and he needs water. Then, die glas water would mean that you have poured a specific glass of water, and you want to get this glass of water that you have to the thirsty person. Mostly likely this is the only glass of water and so you see that it is very specific. When saying 'n glas water, you might have to find a glass (any glass) and pour water in it.
'n -- Always in lower case. If you start a sentence with 'n, the next word will be capitalized. e.g. 'n Man met 'n masker is in die bank.