Use search terms that describe the perfect result -- e.g. "history of the war on drugs book"
Pay attention to results that come from university archives -- often schools have special collections that could be relevant to your research. (See examples below)
Be on the lookout for museums -- they may have collections relevant to your research. (See examples below)
Google will find you a wider number of sources (which may or may not be helpful!).
Google Scholar will narrow your focus to scholarly / academic articles.
You may find information on collections and other materials through newspapers. Some newspaper databases available at UC Merced include the following.