As a Morgan Kaufmann publication, this leans more toward an academic text book than what you might think to be a straight how to book on casual game design. It is a study of the genre and provides discussion with analysis of various aspects of actual products and their appeal. It covers pointers for designing a game, role of the designer, designing gameplay and interaction, and follows with review of various categories of casual games, such as matching, sorting, seeking, constructing, etc. By attaining this level of understanding you will be better prepared to design, develop and deliver a successful casual game.
For casual games, you can think of Windows Solitaire/Klondike, Bejeweled and the other PopCap games, even some of the Wii titles, and of course many iPhone game apps. Plants vs Zombies is one of my favorites. They are usually simple to learn, can be easily fit your schedule - really just pick up and play. It has grown to be a formidable segment of the gaming market, contrasting with the traditionally "serious" gamers' draw toward long campaigns, complex controls or involved interaction, and big MMOGs. The text includes multiple examples of the various games types to help illustrate the concepts presented.
If you were just throwing a game together for fun, you may find the material to be pedantic. However, if you are a student studying game design or a developer hoping to create the next hit game, it would serve you well to give this book a read if it's not already on your shelf.