Here's a list of some good leftist books I've come across over the years. Click on the titles for the PDFs. It's a somewhat arbitrary list, but I tried to keep it confined to fairly easy-to-read books, not overly theoretical or abstruse ones that might leave the beginning reader bewildered. I've been unable to find PDFs of certain excellent works, unfortunately. You can also consult the Marxists.org website, a treasure trove. Or check out the extraordinary footnotes to Chomsky's Understanding Power, which have informative excerpts from hundreds of books.
Douglas Blackmon, Slavery by Another Name: The Re-Enslavement of Black Americans from the Civil War to World War II
Terry Bouton, Taming Democracy: "The People," The Founders, and the Troubled Ending of the American Revolution
Samuel Bowles and Herbert Gintis, Schooling in Capitalist America: Educational Reform and the Contradictions of Economic Life
Aaron Brenner et al., eds., Rebel Rank and File: Labor Militancy and Revolt from Below during the Long 1970s
Mike Davis, Prisoners of the American Dream: Politics and Economy in the History of the U.S. Working Class
Amy Goodman and David Goodman, The Exception to the Rulers: Exposing America's War Profiteers, the Media that Love Them and the Crackdown on Our Rights
Jesse Lemisch, On Active Service in War and Peace: Politics and Ideology in the American Historical Profession
Peter Linebaugh and Marcus Rediker, The Many-Headed Hydra: Sailors, Slaves, Commoners, and the Hidden History of the Revolutionary Atlantic
Staughton Lynd and Daniel Gross, Labor Law for the Rank and Filer: Building Solidarity while Staying Clear of the Law
Michael Mandel, How America Gets Away With Murder: Illegal Wars, Collateral Damage and Crimes Against Humanity
David Montgomery, Workers' Control in America: Studies in the History of Work, Technology, and Labor Struggles
Kim Phillips-Fein, Invisible Hands: The Making of the Conservative Movement from the New Deal to Reagan
David F. Schmitz, Thank God They're on Our Side: The United States and Right-Wing Dictatorships, 1921-1965