If you're looking for a book that's both easy and stimulating to read, but that discusses a lot of mathematics in reasonable detail, I highly recommend the novelist David Foster Wallace's Everything and More: A Compact History of Infinity.

It's probably the best book on mathematics I've read. It's not a textbook the way the Feynman lectures are, but it's stimulating and a good read. Other books mentioned like Visual Complex Analysis or Courant's book are dry and take a lot of effort to get through. Some of the older books mentioned may be great (I've found many older textbooks much clearer than more recent ones), but I personally haven't read them so I can't make a recommendation there.

You can also check YouTube videos/courses e.g. one I found great was MIT Professor Gilbert Strang's Linear Algebra course -- his videos are easy to follow, stimulating and clear.