JumboSearch and BLC WorldCat are the two most important places to search for books and recent publications.
To find primary sources in JumboSearch, use terms such as: personal narratives, memoirs, memorials, correspondence, letters, diaries, etc. Of course, look for works by individuals of the time.
Books written in Europe will tend to use "First World War" and Americans tend to use "World War I", so you can get an idea from the title the perspective of the book right away.