This is the word by word translation of an english expression In Esperanto, it's too heavy, too long. I would use "plezure" or "kun plezuro" with other words: Mi renkontis vin plezure. Estis plezuro renkonti vin.
Why is it not "la mia"? Or is it just a matter of choice to use "la" or not?
"Libro". There is no definite article "la", so I know nothing about the book.
"La libro" The definite article "la" is used to tell that the book is already mentionned, I know its title.
"La mia libro": Since the book is mine, I surely know it, so I could use the article "la".
"Mia libro": But the word "mia" already includes the sense of "la". So most people use this form without "la".
@BertShumann Thanks for the good explanation. I only asked this question due to the fact that Duo has taught us "la mia", in earlier lessons, to mean "mine".