All books are great, depending on the person that reads them. I wasn't entirely happy with some books, especially when they have weird or sad endings.
Here are a few that didn't satisfy me. I did write something like this before :
Here are the reasons why I wasn't satisfied with The Deadly Dare Mysteries - By Malorie Blackman.
PS : This is just my opinion, don't get mad.
1) Deadly Dare
In the end, Angela's brother Tom went to jail. He turned out to be a good guy at the end of the book but he still got arrested. Angela could have happily lived with Tom for the rest of her life but for some reason gave proof to arrest him. I always kind of liked Tom from the beginning. He was nice to his sister and everything he did was for her good though he got a bit carried away. That was a sad ending.
2) Computer Ghost
I prefer stories which can be possible in real life. Even though the adventures in books like Famous Five or Murder most Unladylike are far fetched, they are possible in real life with extreme coincidences, but Computer Ghost disappointed me. The story ended saying that the ghost of Paul Driscoll was helping Theo, Ricky, Angela and Bullet the entire time. I want a solid no-ghost-everything-is-possible story. I'm not saying that horror and ghost books aren't good, but I didn't expect it here.
3) Lie Detectives
I never liked Darius Marriott. I always knew he was an evil guy... to some extent. I also love the character of Bullet aka Toby. I don't know what happened to Bullet in the middle of the book, the character change, and I didn't like it one bit. But that's all done. What really bothers me is how Darius has Bullet completely tricked in the end. I mean, we know it won't end well. Darius cares about only himself and will do anything to protect himself. Am I entirely happy about this ending? No.
I haven't read the next books in this series yet so I didn't know if something is going to happen to change these endings. But for now, I'm kind of not happy. I'll keep you updated as I read the next books.