"Business Process Execution Language for Web Services" is a good tutorial and reference for BPEL. The authors state that the book is targeted towards current web services developers and architects. Appropriately, the book assumes knowledge of XML, UML, and of course, web services.
The book was written by three authors and the chapters are separated by author. One author wrote chapter one, which includes an overview of BPEL, concrete examples of workflow and how BPEL fits into the big picture. The main author wrote chapters two through four, which flow well and teach BPEL through a running example, along with describing Oracle's BPEL tools. The third author wrote chapter five, which describes Microsoft's BPEL tools. The book ends with a clear, concise syntax reference. While all the chapters provide valuable information, the first and last chapters seem disjointed from the remainder of the book.
There are plenty of diagrams to show architecture and flow, something very important in BPEL. The meat of the book teaches BPEL, something it does very well. The examples in chapters two and three gradually grow in complexity and build on each other. When describing Oracle and Microsoft's tools, appropriate screenshots are provided. I recommend this book for anyone starting out with BPEL or just looking to learn it.