For the most part, you can read Crystal Cave (Sueño Bay Adventures, Volume 5) by Nancy Deas and Mike Deas separately from the others in the series. However, I am assuming it probably is best to start with book one, Shadow Island (I wasn’t sure who/what the pet/friend of our main character was). The story itself is not necessarily a new one, but it is told in a fun way that keeps you reading. Good illustrations and a nice fantasy/mystery/realistic story.
The idea is that the island the characters of the story live on is a bit magical. People are healed a little bit easier and the mysteries around that are strongly kept. Of course, when you have a couple of loveable old coots who like to talk and give out really good hot chocolate, sometimes secret clues are leaked. Especially after Ollie’s grandfather gets ill and must leave the island. When Ollie and his friends go looking for the mother load, the Mother Crystal itself, things get convoluted and more than the tides are acting funny.
This is a nice story about friendship, family and working together. The overall of things is hopeful, but has the “spooky bad things might happen” too (but is perfectly tame and at best is a Y in rating). Our characters learn a lesson (magic has a funny way of making you think you have what you want, but is it what you need?) and have adventures too. And while things are for the most part realistic, there are some real magical elements and characters, too. This allows for multiple genre readers to enjoy and most ages at least 8 to younger 13-14.