The book starts with a detailed discussion of differences between relational and document-based databases. The author makes it clear that Mongo DB is much more flexible in terms of schema design for many real-world applications.
Another interesting topic is mimicking transactional behavior in MongoDB. The author demonstrates how to implement transactions in MongoDB through a simple book-keeping application.
In subsequent chapters the book describes many interesting features of MongoDB through a series of real-world examples, such as designing a database for e-commerce site, for a CMS, etc. Each subsequent chapter reveals a new feature of MongoDB. For example, the chapter about CMS explains how to store binary data (image) in MongoDB. Chapter about operational intelligence shows how to use Map-reduce framework which is built into MongoDB.
The use of many real-world but simple examples is strongest feature of this book. Many web developers have encountered the problems mentioned in this book (log parsing and analysis, storing images, etc.), and the book shows that MongoDB can solve most of those problems in a simple and elegant way.
To summarize, the book makes it very clear that document-based databases is the future of data in the web.