This is the seventh book in this series and so far, though I've not read all the previous six, it's my favourite. Laura, author and main character ages from 14 at the beginning of the book to very nearly 16 at the end. Calm, sedate Mary goes off to the school for the blind in Iowa and while her family might miss her, it doesn't really affect the rest of the book.
This is the book that takes a step away from the intense focus on the immediate Ingalls family and starts to really interact with others. Laura and younger sister Carrie attend the local school and form friendships with other children there. The family again spends the winter in town, though this is a far milder winter than the previous one they experienced (see The Long Winter), and as a result they are far more social. At least part of this may be due to the fact that there are far more families in the area than there were the previous year.
Anyway, it's a classic series that you really should read. It's also worth reading from an adult perspective, in my opinion, if you enjoy historical stories.