Some would say, "Don't judge a book by it's cover," but first impressions mean a lot. I agree with the Magical Minds, it's important to "Take a look" at a book before choosing to read it.
If the topic of the book isn't interesting to you, you probably wont like the book. If you aren't that into sloths, a book about sloths might not be for you (but, then again, good books will creep up on you).
A Just Right Book is not difficult to understand. You know most of the words, but occasionally come across some words you don't recognize. Some people use the five word test.
The Five Word Test: open a book to a page in the middle. Read the page and count how many words you do not know. If there are more than five words you don't understand on one page, it's probably too hard.
There is no problem with reading an easy book. I do this all the time. My guilty pleasure is reading young adult novels in bed. They are, what I call, "yummy-eat-em-up" books. My wish is for each Magical Mind to have an easy book they can curl up with. But, an easy book doesn't challenge the mind to grow.
There is also no problem with occasionally reading a challenging book. My professional development books can be quite difficult, full of challenging words and concepts. I read these books slowly and carefully. Although they are engaging, I wouldn't say it's fun. Challenging books are often discouraging.
The moral of the story: We grow and learn the most when we read books that are Just Right.