First of all, you need to read the book thoroughly and carefully by paying full attention to the overall tone, plot and characters of the book which will help you to summarise it in your overview.
While reading the book, you should note down your observations which will help you to collect the most important points of the book and it will be easy for you to write the overview in an authentic manner.
After giving a thorough reading to the book, you need to compile a summation or summary of the plot by taking guidance from your notes that you have written while reading the book.
You should decide whether you should write the overview of the whole book or want to focus on different chapters. However, writing an overview that covers the whole book is preferred by a majority of people because the readers want to know about the whole book.
The most important principle that you should follow while writing an overview is that stay as brief in writing your observation as possible. Conciseness is the most important element for writing a decent book overview.
You should deeply observe the overall plot of the book and also the way of expression of the writer. Make sure you present these things in your overview in a compact manner. You should also include your own thoughts about the book in two or three paragraphs.
Include the cultural and historical context of the book in your overview which will help readers to understand the characteristics of the period and thus will enable them to read and understand the books to a greater extent.
In the overview, you should also write down the message or theme that the author wants to convey in the book. Organise your overview in a manner that looks appealing to the readers.