I really thought that The Ghost's Grave was a fun book. In some ways it's a typical, "Mom-and-Dad-ruined-my-summer-by-making-me-go-stay-with-a-weird-distant-relative" story, but somehow this story was believable-lite, rather than completely in the realm of utter kid lit fantasy.
The author accomplished a believable-lite story by giving us a main character who was sympathetic and smart, and therefore believable, by paying attention to detail and realizing that the strength of a story can hinge on those details, and by refusing to rely on deus-ex-machina tactics in order to get our hero out of sticky situations. There was adventure and suspense which kept the story moving, and there was a bit of heart to the story, which really helped to explain what motivated the main character to behave as he did. He had a set of values and ethics that gave him, and thus, the story credibility.
Now, the story was written for children, so it does have some elements of improbability and fantasy in it, which is where the "lite" comes in in "believable-lite," but that's ok. It's balanced in this book, which makes the book feel like it's written with a more mature 5th-6th grade audience in mind, but it takes into account that kids in those grades are still kids and want some escapism in their stories. I would feel totally comfortable recommending this book to my 11 year old niece. I think it would hold her interest and keep her entertained. And even if it didn't, I don't really care. It entertained me, so I'm content!