Personally, I think the qualities of a good book are many, and diverse. To be good, I think a book should relate to life experiences, give the audience something they can relate to and empathize with, to help them learn something that will help them in their life. Yet, the book should do this in such a way that not only does it act as a window into the human world, yet also offer escapism. It should have a character that the readers can identify with, in a way that they imagine what it is like to be the character themselves. Though, the character should be normal enough to identify with, yet special enough to go on adventures (in fictional books) and unique in their own ways.
Another factor that comes into play in the qualities of a good book is the fact that it should captivate the reader; should be wholly absorbing and keep the readers not only reading the first few pages but reading till the very end. I myself have left many books through the middle, seeing as they had lost my interest. A good book should have a very unique original plot and story line. The readers should be able to enjoy what they are reading; and to do that the story must be original, since I (and presumably many others) find no pleasure in reading books with similar or identical plots of situations. Not only that, but the plot must have twists and tensions to make the book more appealing to the human minds analyzing it. A good plot never comes without tensions in my point of view- it should be a whole package. The book should keep the readers on edge, reading and turning page after page wanting to know what will happen next. That, for me, is the post important quality a book should have. It is quite easy to get the interest of many people into reading a book, but to have the readers finish it anxiously till the very last page is something that takes skill, talent and quality, and not all have those critical traits. Thus, tensions to say the least, are a must,