Unless you know the title and/or author of the book you are looking for, history topics should be searched by Keywordto maximize the number of references to a topic.
The word "sources" appearing next to an entry is usually a good indication that the text is at least in part a primary source. You can increase your chances of turning up primary sources by adding terms such as "sources," "letters," "diaries," "personal narrative" etc. to your keyword search term.
Out of the Blue: The Story of September 11, 2001, from Jihad to Ground Zero by 
The 9/11 Encyclopedia by 
The Landscapes Of 9/11 by