"This is one of those good books that actually treat the Psalms for what they are: prayers. Many introductions and commentaries neglect this, treating the Psalms as though they were history, instruction or whatever. While history, instruction and a lot else is in the Psalms, their primary purpose is that we may pray after God, speaking His words back to Him.
The best introduction to the Psalms is probably to begin praying them, perhaps with the help of a good prayer book. But if someone's really green, this book will provide an understanding of the different types of Psalms, different human authors within the Psalmnody, and an overview of some of the difficulties we perceive within the Psalms.
My big complaint with the book is Bonhoeffer's occasional over-emphasis on his perceived need to pray (using the Psalms in this case) against an also-perceived wrath of God. This reflects an unfortunate -- in my opinion -- emphasis, one likely stemming from his Lutheran background. But it's a relatively minor issue with the book.
This book is a quick (it could easily be finished in an hour), easy read. The book's also divided into sections which could be used as subjects for daily meditation or for a series of classes."