I have always enjoyed reading Martin Aspeli's contributions to Plone documentation at plone.org and have found his articles clearly written and extremely helpful.  His book, Professional Plone Development, continues in the same tradition with straight-forward explanations.  This book is aptly titled. It's not a first book to read for Plone, but is for people already familiar with the Plone framework.  The intended audience is definitely web site developers.

This book tells you how to do things you'd never be able to figure out on your own.  It's a cookbook for how to get things done and really gets down to the nitty gritty.  Plone 3 is complex, but here it is carefully explained with a clear presentation of ideas.  So many things are new & different in Plone 3 that it's hard to wrap your mind around it.  With this book at least you'll be speaking the language.  There are many overviews of Plone 3 functionality and it seems to have touched on every subject related to Plone 3.  It's very comprehensive.

Lots of practical ideas and great tips are included - like using a code management system even for small or single-developer projects.  Aspeli highly encourages testing and shows how to implement unit tests.  At the beginning of the book the reader is presented with the history of Plone.  It is a worthwhile background and will make you feel comfortable about the Plone community.  The chapter explaining Zope clears up a lot of confusion about the internals of how Plone works.  Since Plone is an *application* for Zope 2, sometimes it is difficult to remember what part is Zope and what is unique to Plone.

A case study/sample project is used to present examples and make the details easier to follow.  This book is loaded with sample code and even includes an online resource for the code examples in the book.  Aspelli gives friendly syntax examples for Windows as well as Unix.  This book also serves as a great reference and lists lots of resources for learning more, giving locations of the original documentation for further reading.

Overall you'll find clearly written instructions for developing your own site and a great reference book that will bring you up to speed on Plone 3.