Statement of purpose book reference
As mentioned in the comments by Anonymous Mathematician, there won't be a problem with mentioning a book that you read in your statement of purpose. If your statement of purpose has a list of references, you can include it there and just use the standard referencing style to cite it. If you don't need a list of references, then just cite the book in the text, e.g., "Reading the book 'A nice mathematics book' by , I found that...".

The difficult part will be to make citing the book really worth putting it in your statement of purpose. Which books someone read is probably not as important for computer science students as it is for students of literature. Think about the following questions, as examples:


How did reading this book influence your decision to apply for graduate studies?
In which way will your planned research build upon what you learnt in this book?
Does the book define research problems that you are planning to tackle?


If you cannot write something along these lines when referring to this book, then I am not sure that it would be worth writing that you read it.