If you want to use foreign language literature to learn the language, the texts should be somewhat above the current level, i.e. not too easy, but not too difficult either. The grammar should also not be too easy, but should require some attention, because then a new word or term is more likely to stick. As a rule of thumb, about 95 per cent of the vocabulary should already be known. In general, you should choose reading on topics that interest you outside the language.
If the text you have chosen is written in a more sophisticated style, you should not immediately look up unknown words, as this will only spoil your enjoyment of the story. Most of the time this is not even necessary, because after you have encountered the word a few times, its meaning often becomes clear from the context. It is estimated that you have to encounter a word at least six times in different contexts to grasp its meaning, although it is quicker if the context is very informative.
In general, voluntary, self-selected reading is more effective than guided learning, although it can be useful to read a beloved series of books, as the familiar themes and repetition of similar vocabulary can aid comprehension. However, the most important thing about a learning read is to stay interested and enjoy it.
A good learning opportunity can also be a well-known TV series in the original sound, possibly with subtitles in the native or foreign language. For starters, it is often recommended to listen and read at the same time, as this helps to divide the sound stream into words and thus also reduces the risk of mispronouncing what you read in your head.