Sunnikunnoth, It actually depends on what language skill (written or spoken) that you're talking about and the age of the student. For little ones, of course, total immersion in the spoken target language is key.
For accent reduction in SPEECH of older learners, total immersion in listening to the target language is also important. In a language like English where one letter does not necessarily represent the same sound all the time, reading can actually make spoken language sound stilted. The word, "often," for example as you probably know, is actually pronounced "offen" in the US. "Did you eat yet?" actually sounds like djeet yet? We don't pronounce the "h" in question words like "When," "Where," and "Why". So reading of English doesn't always help with spoken English if you're wanting to sound like a native speaker. Of course spoken language is dynamic and constantly changing and you have young people and even celebrities uttering the HORRIBLE sentence, "Me and him went out last night." UGH! The speech of newscasters like Diane Sawyer and Brian Williams are more acceptable styles of standard spoken English.
Often my students will watch captioned TV, write down vocabulary,etc. that they don't understand and we discuss it.
To improve writing skills, I think it really doesn't matter. Both fiction and non-fiction will be equally good role models. However, dialog in fiction will provide more idiomatic, slang and colloquial speech and conversational styles.
Non-fiction writing, such as essays and edited articles written by professionals will provide proper written English as well as proper use of semi-colons, commas for introductory phrases, compound sentences, etc.
The most difficult part of written language for students I have tutored from other countries is subject-verb agreement and other syntactical features like comparative and superlative cases of adjectives. It's easy to tell from writing whether a student is emerging in written language in English or has mastered it. (I would imagine the same is true for other languages).
There are numerous ESL "salon" and websites with free online exercises in subject-verb agreement, adjectives, etc. etc. etc.. I have a few on my website, but there are hundreds of activities created with Half-Baked software (Hot Potatoes) that was developed out of the University of Victoria. If you google them, you can get to some of the grammar practices. Oh, and I've seen great ESL articles here on HP. Best Wishes