Mathematics is not an easy subject. Some people say it is, but they are wrong. Even the best mathematicians eventually encounter problems which test their ability to the full. When learning advanced topics it is not uncommon to spend hours trying to solve a problem or understand an exposition, and often one needs to talk it over with a teacher or colleague. If you only have a book to consult, you need a very good one, and this is such a book. It is carefully constructed, the subject is developed logically, it has many examples, varying from the straightforward to the demanding, and it is written by someone with a huge enthusiasm, even a passion for the subject.
It is, of course, not an easy book. It took some of the brightest people in the world about two and a half thousand years to develop the calculus, and we have only had it for less than five hundred. If you read this book, you will be on the way to understanding what mathematics is all about. Do not try to do all the examples, but make sure you can do a fair sample.