From the library homepage, click on Advanced Search. Use keywords for the period or event(s) you're looking for along with the type of primary source(s), while pulling down the BOOKS option from the Material Type menu off to the right:
That search yields plenty to browse through.
Clicking on one record then allows you to examine the Library of Congress Subject Headings which could be very useful.
For Instance, clicking on the link for "United States -- History -- Civil War, 1861 - 1865 -- Personal narratives" yields a wealth of material, much of which is available online: