The immune system IS amazing. My ONLY goal in the brief time we have to talk about it is to make you want to learn more about it. Whilst there are several courses here that you can take there is also a book that I can highly recommend.
If you want something that falls between the single chapter in an introductory biology textbook and the hefty tome of an immunology text then I can recommend 'How the Immune System Works' by Lauren Sompayrac. If you want to learn more about the immune system, including some cutting edge stuff, but don't want to take a whole course then this impressively slim book (144 pages for $30) is the way to go.
I'm linking to Amazon so you can see the impressive reviews this book has collected: 32 of the 39 reviews are 5 star. There are several other books in the series. 'How Cancer Works' by the same author is also pretty good. These are slim books, it isn't hard to read a chapter a day and in about a week you can have a pretty good understanding of the immune system or cancer.