The answer can be both: "some books" are "objectively better than others", and a "matter of opinion."
Points on what I look for in books and what I look for when I read them.
1. In the preface, foreword, or introduction, the goals and intentions are explained. This is a key point in whether a non-fiction book is "good". Has the author met the goals? Does the author have documented facts, or is the information a legend?
2. Is the book dry and dull, void of breathing life into the historical people? A non-fiction book can be entertaining, and not an academic treatise.
3. Has the author written a book with an agenda, or bias? This can be out-right evident, or it can be sneakily added when we least expect it.
4. Is there a thorough Bibliography and notes section in order to state where there facts came from, as well as give credit to others.
5. An author's enthusiasm transfers to the page, making the book more interesting and engaging.
6. Has the author given information at such a quick pace that not only my head is spinning, but there is no way I can follow what he is writing? I take notes on every book I read. On a few occasions, a book has filled my head with information to a point I feel my head is about to implode or explode. Taking notes is a life saver in order to keep pace with and understand what I've read, it is also a great way to help when its time to write a review.
1. Has the author brought life to the characters? I've read some books where too much descriptive information is given about its characters leaving no room for imagination; on the other hand, other books have given few details which make its characters anorexic.
2. Does the ending tie-up quick? Is it predictable? Does the book end with a hint of a second book?
3. The pace of the story, is it spread out thin? Does it have a rhythmic melody, like in a score of music, with crescendos, fortes, and rests?
4. Is the book of interest to me? Is it a genre I enjoy reading?
5. While reading the book, does the author interject something into the story which is a shocker? There have been books in which I wondered if the "shocker" is done on purpose, to cause a sensation, or attention?
6. In a historical fiction book, which parts are true, and which are embellished, or completely untrue?
7. I love strong characters, but do not need to like them. Milk-sop characters should only be in the background.
8. When I'm finished reading the story, will I take the story and characters with me? There are stories and characters which are still haunting my mind.