Yesterday I reviewed a book on vtiger, vtiger CRM Beginner's Guide, which is a beginner's guide to installing and administering that package. The current book by Aspeli is a big contrast, and not because its topic is running a CMS. Aspeli writes for a developer or administrator already well versed in Plone. The text is not about using a nice easy GUI to do straightforward administration or customising tasks. Instead, it focuses on hard core programming issues. There are relatively few screen captures of a cute GUI, as an empirical indicator of this.
Informally, the book is a sequel to Aspeli's text on Plone 3 - Professional Plone Development: Building robust, content-centric web applications with Plone 3, an open source Content Management System., which came out in 2007. The 4 intervening years have meant that Plone 4 has significant enhancements over Plone 3, and if you are still using the latter, consider at least perusing this book to see how you might improve the customising of your website. Likewise, Aspeli co-authored a recent  book on Plone 3, Practical Plone 3: A Beginner's Guide to Building Powerful Websites. If you founds its advice germane and useful, this could be another inducement to read about the latest Plone.
The book has several parts, roughly in order of increasing difficulty. The simplest is part 1, the first 3 chapters, which is an overview and explains the installation steps. Let me just say candidly that if you can't get through this part, then barring an error in the downloaded code, you shouldn't be reading this book at all, because not only can't you get to the rest of the book, but even if you could, it will likely be over your head.
Part 2, chapters 4 to 8, explains how to use themes in Plone. A lot of Plone administrators will only need this part. There are many ways to alter the look and feel of Plone and of many of its other runtime characteristics. Changing the skins is the quickest way to present a unique look to your website. Separate from user interface visuals, the other main idea in these chapters is understanding the different roles in Plone. Each role is a job, like member, manager and reviewer. The table on page 109 summarises these well. Plone's developers have tried to factor out as many parameters as possible, and have these alterable by you, often via a Plone-provided window for simplicity.
Part 3, chapters 9 to 15, is much harder. This is where you go to code new functionality, that is outside the default scope of part 2. The current book might be seen as an extension of Zope, where the latter is a successful web application server that is also open source. There are many references to Zope features in the narrative and in the code snippets. Chapter 9 is devoted to the key concepts of Zope. As a practical matter, it would greatly aid you to be already facile in Zope, so that chapter 9 is merely review. And perhaps consider getting an accompanying text on Zope, like this one - Web Component Development with Zope 3. It transpires that to write new code, you need to understand the enclosing framework of Zope, because you might [actually probably will] have to step outside the current Plone 4 environment, since that is what you may want to modify.
Part 4, which is the rest of the book, chapters 16 to 18, is easier than part 3. Here you tackle server deployment issues. No actual programming is likely needed. This section is mostly for the Plone administrator or the system administrator of your computers and network.