Books are like pizza: Even When It's Bad It's Still Pretty Good. There is always something in every book that's worth the time investment. At least I've never came across a book I wouldn't have finished no matter how bad it was.
But truly great books are really hard to find. I've read many in my life, but it hasn't come easy. For every great book I have probably read a hundred just good books. It's hard to know whether a book is great by it covers, or reviews, or suggestions by others.
I've read books that have gotten great reviews, books that have been recommended to me by others who I know enjoy similar books as me. I have actually also picked up books purely due to their covers. Sometimes those have turned out to be great read, other times the expectations have made them feel above average even if they were actually good, but not great books.
The only way to find out if a book is great is by reading it through. The closest I've ever been abandoning a book turned out to be one of the greatest in the end.