Building the American Republic, Volume 1: A Narrative History to 1877 by Harry L. Watson (
faculty profile,
Wikipedia), a professor at the University of North Carolina, is the first volume in a comprehensive overview text meant to provide an in-depth introduction to the subject, exploring assorted topics and issues which contributed to the formation of the US, free courtesy of the University of Chicago Press.
This is their featured Free Book of the Month for January, and is also meant to be a permafreebie, as part of their Open Access effort to provide accessibly-written peer reviewed textbooks suitable for introductory college and advanced high school courses (and presumably also home study).

Thus, it's DRM-free and technically, volume 2, written by Jane Dailey (
faculty profile), an associate professor at the University of Chicago, which covers up to the modern day through the 2016 US elections, is also available free as well.

:
Currently free @
the dedicated site project page where you can download both volumes DRM-free in a choice of ePub/Mobi/PDF and also read online and also browse some resources for instructors.
And for completionism's sake, linkage to their
regular promo page and the catalogue pages for
Volume 1 &
Volume 2.
Description (for Volume 1)
Building the American Republic combines centuries of perspectives and voices into a fluid narrative of the United States. Throughout their respective volumes, Harry L. Watson and Jane Dailey take care to integrate varied scholarly perspectives and work to engage a diverse readership by addressing what we all share: membership in a democratic republic, with joint claims on its self-governing tradition. It will be one of the first peer-reviewed American history textbooks to be offered completely free in digital form.
Volume 1 starts at sea and ends on the battlefield. Beginning with the earliest Americans and the arrival of strangers on the eastern shore, it then moves through colonial society to the fight for independence and the construction of a federalist republic. From there, it explains the renegotiations and refinements that took place as a new nation found its footing, and it traces the actions that eventually rippled into the Civil War.
This volume goes beyond famous names and battles to incorporate politics, economics, science, arts, and culture. And it shows that issues that resonate today—immigration, race, labor, gender roles, and the power of technology—have been part of the American fabric since the very beginning.