Anything in this post with a spoiler tag WILL contain some degree of spoilers, but I will try to use just simple answers.
How many volumes is Esprit?
Eight.
I hope the ending isn't rushed... (don't worry, I don't tell how the manga actually ends in this "spoiler")
Spoiler
Will we get to see Cage again?
Spoiler
Who is Mio... really?
Spoiler
Does Age ever become useful?
Spoiler
What is with Canary and her "read the atmosphere" business?
空気を読める (kuuki wo yomeru) literally means "to read the atmosphere" but it truly means "to be able to read the situation or mood around you". However, that doesn't really explain the significance Canary really seems to bring to the phrase. As a mermaid reading the atmosphere, or air, should be rather difficult as they spend most of their time in the water.
Will we see her again?
Spoiler
Do any of the main characters die?
Spoiler
If you have any questions you would like to add to this, feel free to ask them in a reply. I don't promise I will answer all of them, though. =P