I'm talking about non-learning books.
Let's say you speak three languages. For this example I'll use Swedish.
Let's say you know how to speak Swedish, English and Russian
Your native language is Russian. You speak/know English on great level and your Swedish is not that great but you are still doing all right.
So let's take the book "The Girl With The Dragon Tattoo"
The book is originally written in Swedish. And the book is translated in Russian, English and Swedish.
-Read it in English cause you want to read it in a foreign language but you still want to understand all of it
-Read it in Russian cause you want to enjoy the book in your native language which you know best and which will make you enjoy the book most
-Read it in Swedish so you can experience the book as it was written but risk not understanding everything while also using this as a way to test and improve your knowledge