To accompany my video lectures on gametheory101.com, I have created a textbook for basic strategic and extensive form games. Since students needs vary depending on the class, I offer two versions of the textbook. The first consists of only the lessons on strategic form games. You can find it on Amazon here for $0.99. The second book has two additional chapters, one on extensive form games and one on advanced strategic form games. You can purchase it here for $3.99. Unless you only have a passing interest in game theory or are taking an extremely easy introductory class, I recommend you buy the bigger version.
Note that they are only available for Amazon Kindle. However, if you do not have an Kindle reader, you can download a Kindle interface from Amazon for your PC, iPad, iPhone, iPod Touch, Blackberry, or Android for free. The PC version is available here; for handheld devices, check their respective app stores.