I read that book recently, and it would be difficult to give a review without filling 2 or 3 pages
I'll instead try to answer your questions.
It's a work of fiction, and I may be wrong, but I doubt that Hesse was enlightened, so it's not Buddhism.
Why is Buddhism happier? In the book the protagonist Siddhartha starts off as an ascetic but is drawn towards a beautiful woman and material greed. This eventually makes him suffer, which is why the Buddhist life is happier.
The book is very short and a good read - it's interesting and thought-provoking. Also it's free of charge, being out of copyright, I found a copy on manybooks.net and would recommend it.