I highly recommend checking out https://www.raywenderlich.com.
When I first started iOS programming (before Swift), I used this website to learn some of the fundamentals. The free tutorials are informative and useful. The site paid tutorials, which go deeper, but I have never used them.
However, they site also offers books on a variety of topics, including Core Data by Tutorials | raywenderlich.com. I bought this particular book because it goes into CoreData integration with iCloud. You might find this useful.