To summarize The Dialogic Imagination, by philosopher and literary theorist Michael Bakhtin, you should consider form and structure, context, argument, themes, and any other aspects of the book that seem relevant to understanding its place in literary theory and in the author’s body of work.

Firstly, it would be worth noting that Bakhtin was a Russian philosopher and literary theorist. His main areas of work were the philosophy of language, ethics, and literary theory. As such, this book continues and furthers his work in these areas.

It should also be noted that this book contains four separate but connected essays, which had originally been published separately, prior to the 1975 publication of this book. The four essays contained in this book are considered crucial to understanding Bakhtin’s literary theory and to introduce and explore some of his most important contributions to the field.

Your summary should discuss the contents of each essay, by introducing the main theme and argument of each one. The first essay, “Epic and the novel,” focuses on defining the novel by contrasting it to the epic. The second essay, "From the Prehistory of Novelistic Discourse," focuses on the history of the modern novel and explores how different texts in history have been crucial to its development. "Forms of Time and of the Chronotope in the Novel" is the third essay, and this is where Bakhtin introduces and explores the concept of chronotope. "Discourse in the Novel," the fourth and final essay, discusses the philosophy of language and particularly the concept of heteroglossia.

Finally, it would be worth paying some attention to the the terms "chronotope," "heteroglossia," and "dialogism." A summary could explore how Bakhtin’s arguments represent a significant shift from the theories of other philosophers of language, like Saussure, and perhaps discuss how he connected the philosophy of language and literary theory to society and social hierarchies.