There are a total of four different buttons, but only three of them are choices for Projects and a different set of three for Contexts. There isn't much utility to showing completed actions in Context view on the iPhone/iPod, for example, and similarly if you are viewing a single project, showing only the next available action doesn't provide much utility either.
The difference between Available actions and Next Actions is not complex. A Next action is the next step you can take to advance a project toward completion. An Available action is an action that you can do now without doing something else first. If it has a start date/time, that date/time is in the past. It is not in a project or context which is on hold or dropped. For a sequential project (or a sequential action group), the Next action and the Available actions are the same -- the top of the list. For parallel projects/action groups and Single Action Lists, you have multiple choices as to what you can do next. For parallel projects/action groups, the top of the list is presented as the Next action, and all of the available actions are presented as Available actions. For Single Action Lists, all of the available choices are presented both as Next actions and Available actions. The thinking is that there is no connection between the items (except that they are in the same list) and so no preference is implied by the ordering.
While you wait for the book to arrive, you might investigate whether your library has an electronic copy you can read online, like mine does.
I did get my iPod back to English, though for a while I had an extra button on the keyboard. Once you turn on another language that uses a different keyboard arrangement, even if you only have one language enabled, you'll get the button for switching keyboard arrangements until you go back to the International preferences and select the keyboards you want available (Tastaturen, right below Sprache).