Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Reflective
For a thirteen-year-old, Isabel is incredibly observant and has an amazing memory. As a result, the language of Chains is full of her assessments of the events, people, and choices in front of her as she reflects on her views of them. She learns quickly in her dealings with Madam, for example, to "[keep] careful track of her the same way as I used to mind the neighbor's bull" (8.3). This description gives us a clear look at how the book's tone mirrors Isabel's thought processes and decisions.
The same also goes for the plans Isabel makes to protect Ruth and herself. Look at how she reasons through her plan to tell Curzon about the money hidden in Madam Lockton's chest:
I could not open the gate, but I had to open the gate. This house was not a safe place. I had to get us out. But there was no way to get out […] The secret of Madam's linen chest was the only key I had. (10.24)
In this passage, the puzzle of having to enter a risky situation to gain her freedom shows the reflective and reasoning qualities of Isabel's thoughts. In short, as our narrator, Isabel has a ot on her mind, and the tone of the book follows suit.