The fact that literary theory is often referred to as just 'theory' should alert the newcomer to its amorphous and unfocused nature. It is no longer concerned just with literature, but with every aspect of culture and experience. It is a theory of theories, a post-modernist stocktaking of the western intellectual tradition.
Culler traces several paths through this boundless philosophical landscape. Seven such paths actually, exploring aspects of language, identity and meaning. These constitute as gentle an introduction as is possible. In fact, it is difficult to imagine a better guide than Culler, with his clear and elegant style and his breadth of knowledge. Although this is not a conventional school-by-school primer, there is a section at the end briefly summarizing the major schools, from Russian Formalism to Queer Theory (yes, you heard right). The author advises that you can read these summaries before, after, or during the main text. I recommend leaving them until after, when they will be a lot more meaningful. Otherwise, they might frighten you off from reading the text itself.
The illustrations consist of a half-dozen or so vaguely relevant cartoons. I suppose, as this series is illustrated, OUP felt obliged to include something, even if the text had no need of it. More positively, this book is blessedly free of the typos that normally bedevil the series.
If you wish to 'dip your toe in the water' of literary theory (and be warned, it is a maelstrom) Culler's book is the perfect place to do it.