As an RDBMS practitioner (MSSQL, PostgreSQL), I was interested in coming up to speed with some of these new alternative data stores. I chose MongoDB initially due the "look and feel" of it's site, documentation, and community. After watching an O'Reilly webcast, I was interested enough to purchase the book.
In subsequent chapters, implementation, administration, and development concepts are covered. There is also a brief internals section that may help the traditional database user understand the inner workings of the MongoDB engine.
It is an enjoyable read, and I expect that this book will continue to be a useful reference after the initial read through, as my experimentation with MongoDB continues.