I read tons as a child (still do), so I suggest many books I liked to my children. For new literature, I used to pre-read everything but can no longer keep up with their reading pace. I can usually get enough of a sense of the book from amazon reviews or library websites that have a synopsis and reviews.
Good reviews also come from friends whose kids are slightly older than yours so they can preview the books for you. Or a helpful librarian at your child’s school or the public library. As well as knowing the plot twists, you’ll want to know how the characters react to events and the general tone of the book. The same events (say a divorce or underaged drinking) can be handled very differently depending on the author’s worldview.
I steer my children away from the instantly popular series type books (like Gossip Girls or Twilight) that you can tell in an instant are bad. Most anything written expressly as “young adult literature” these days is not worth reading. For younger kids, there are so many good books out there, that you can keep them reading non-stop without ever having to go near junk books. Using reading lists like 'Honey for a Child’s Heart", “A Landscape with Dragons” or “1,000 books” you can fill up your library card in no time!