The book is The Assassination of John F. Kennedy - The Reasons Why (1970).

http://www.amazon.com/The-assassination-John-Kennedy-reasons/dp/B00005VGHV

The book discusses DeMohrenschildt's April 1963 remark which was made to Oswald and which referenced the then-recent incident in which General Edwin Walker's home was fired upon. The book, written in a strict timeline format, also discusses other interactions between Oswald and DeMohrenschildt.

It is in the forward to of the book (I think) that the author Albert Newman, a World War II news correspondent, suggests that one other person may have been aware of Oswald's intention to assassinate the president. The author states that he would not specifically identify who that person was, but that his identity would become clear to the reader of the book.

I do not necessarily agree with the author's ultimate conclusion which is that Oswald acted without the assistance of any conspiracy. However, the book is a good read because it relates details such as Cuba's short wave radio broadcasts which Oswald may have listened to on his portable radio.