A proven way to be a better writer is to read extensively. Exposure to correct language through extensive reading in some ways mimics the way you learnt your native language: by unconsciously extracting correct patterns and thus absorbing the grammar of your native language. Then, in school, you learnt to read by working your way through a series of texts of increasing complexity, from a few simple words and simple sentences to a wider vocabulary, longer sentences, and more complex constructions.
If your current reading in English is limited only to research papers and similar technical literature, you are unlikely to make progress in writing better English. This post is about elementary books that are specifically aimed at making you a more competent reader. Two useful series, both published in Britain, are Quick Reads and the Oxford Bookworms Library.
Quick Reads are books by established writers. They are especially written for emergent or reluctant readers and cover both fiction and non-fiction. The books are short (about 130 pages each, no more than 20,000 words) and written in a simple style: they consist of short paragraphs (no more than 10 lines), short sentences (average length is 15 words), and short words (no more than two syllables).
The Oxford Bookworms Library offers more variety and includes abridged and simplified versions of well-known books. They have several types of stories in their collection, ranging from World Stories, which are short adapted stories from writers around the world, and The Collection, which are well-known classical and modern stories in their original form.
Both the series offer audio versions of the full texts, and you may find that you enjoy listening to the books more than reading them. This is also a good way of learning and assimilating the language. Another advantage is that you can listen to the audio books while driving or doing other such tasks, so you won’t need to set aside separate time for reading the books.