These sites have short articles written with relatively simple grammar and vocabulary Orange news Simple news website, check the “quirkies” section for some funny and strange articles. USA Today Very simple American newspaper.
English Club has lots of information, quizzes and guides to help you understand and practice English better. Their forums are a great place to chat with other learners about things that interest you.
Sometimes an exercise book just isn’t enough and you really want to know the details. This is when you need a grammar reference. Practical English Usage and Cambridge Grammar of English are two of the best.
Raymond Murphy’s Grammar in Use Intermediate is one of the most comprehensive books around of grammar rules and exercises. Every student should have this. There is also Advanced Grammar in Use, and while it is very complete I think it is appropriate for an extremely high level (Cambridge Proficiency Exam). A good exercise book in between these two is Cambridge Grammar for CAE and Proficiency.
English Page is a good collection of fundamental grammar rules with explanations, examples and exercises. A good first reference point.