There is so much information and learning material available now that it is sometimes hard to know where to start or what is the best place to go especially if you are just starting out or moving onto the next level.
Well for me the place I always go to first and keep returning to is Objc.io. Its tutorials, reference material and examples are second to none and offer so much in terms of learning and expertise.
They have a set of books which really are invaluable if you are learning any aspect of iOS, both for new developers and more seasoned alike.
I also want to highlight their new project which is weekly talks where you even get to watch them as they code and talk through why they are doing each part. Watching experts walk through their code is always excellent and a great way to learn or pick up new practices and methods.