You'll find most of the US history books in McCabe in the E and F sections. The E section is on the Lower Level. The F section is on Level 2.
Library books are shelved by subject according to the Library of Congress Classification system. So some books relevant to your research may not be shelved with the Es and Fs, because they may be kept with other books on politics, religion, mass media, etc. (Curious about this system? View this list of sub-categories within the Es and Fs, American history.)
You don't need to memorize all of this - just use Tripod to find a call number, which tells you where the book can be found.
Call Numbers in McCabe Library:
If the book you want is only available from H (Haverford / Magill) or B (Bryn Mawr / Canaday), click the Request link.
After logging in, complete the request process to have the book delivered to you at Swarthmore within 48 hours (usually faster).