Personally I’d avoid reading biographies until you have a better grounding in general history. There are very few if any people in history who’ve had lives so fascinating that you can spend 500 pages talking about them without becoming boring somewhere along the line or having to descend into real minutiae that few aside from academics care about. Go instead for the broader histories, then you can narrow it down later.
Since nobody can deny the importance of wars in defining America, you have to get a good foundation there. A former Marine corps officer and journalist named Robert Leckie has written a series of popular one-volume histories of America’s wars. They include George Washington’s War , None Died in Vain (Civil War), and Delivered From Evil (World War II) as well as others on Korea, 1812, etc… While they’re not what you would call scholarly or groundbreaking or controversial, what they are is extremely readable and interesting even for somebody who has no real foundation in military strategy or American history. In addition the battles, he gives biographies of the key players as he goes, including several of the lesser known but very important figures, and makes a complex subject simple to grasp.
For WW2, it’s hard to beat the book or the miniseries version of Band of Brothers , a series so great that even Jimmy Fallon couldn’t hurt it (though I’m sure Tom Hanks had to slap him around to keep him from cracking up). This series literally begins where most war movies leave off, and best of all it’s closely based on a true story. While most war movies (even excellent ones like SAVING PRIVATE RYAN and THE LONGEST DAY) only concern one battle or one character for time reasons, this one follows a company from basic training in Georgia to postwar Germany and addresses things seldom covered in war movies such as replacements, points, promotions, officer changes, etc. and it’s just fascinating.
Lots of good miniseries and documentaries from the brothers Burns including NEW YORK CITY, THE CIVIL WAR, MARK TWAIN, THOMAS JEFFERSON, LEWIS & CLARK should all be available free and easy through Interlibrary Loan. (I’m a librarian myself and ordered all of these with some recent gift money.)
Ditto the above recommendations of THE FIFTIES (excellent book that can be read cover to cover or one chapter at a time), RAGTIME, 1776 (I’ve known several professors who show this movie to history classes), and LITTLE BIG MAN (Custer was my hero when I was a boy, then I saw this movie and was sad they could only kill him once).
Speaking of Custer, I’d read Dee Brown’s classics Bury My Heart At Wounded Knee (about the rape of Native Americans) and The Gentle Tamers (about women of the west, including Libby Custer and Belle Starr) for some quick and easy and interesting reading about the American west. The Conrad Richter Awakening Land trilogy (The Trees ,The Fields and The Town ) are amazing novels on the opening of the Ohio frontier from ca. 1790-1860 and inspired one of the greatest miniseries ever aired.