"Ethnic America" and "Black Rednecks, White Liberals" are also rather politically slanted, but Sowell does what sounds like a better job of objectivity, and presenting data to make his case. He builds up to his political slant as an outlook based on the information presented, rather than the underlying basis for his thesis.

Ethnic America examines various immigrant groups and their arrival in America, their patterns of settlement and activity, and the conditions from which the emigrated and how those influenced their development in America. He looks separately at the Germans, Irish, Italians, Chinese, Japanese, Blacks and others, and compares and contrasts their experiences and performances as well.

This bit reminded me of "Black Rednecks", which briefly touches on these English settlement patterns to establish the culture into which blacks found themselves transplanted. The premise of "Black Rednecks, White Liberals" is to show how the Scots-Irish influence pervaded the culture of the Deep South & Appalachia (even to a degree in the upper classes), and how in the old patterns of behavior brought over from England, we see the roots of many behavioral patterns those in more urbanized regions, outside the South, tend to associate with urban black culture, rather than their Appalachian and Southern origins. IIRC, BRWL cited Albion's Seed more than a bit.

An interesting parallel to your observation of the historical objectivity deteriorating as the history moves along is one I have observed in my own reading of Warren Carroll's "History of Christendom" of which I received the sixth and final volume for Christmas. While I enjoyed the prior volumes in the series, finding only a handful of issues with which to disagree in any given book, I have found things to object to in practically every chapter, coinciding with the coverage of the Civil War and subsequent events, particularly in regard to American history. I think it is perhaps that historical objectivity is harder to maintain the closer you get to events that directly effect you. Once you start getting into the things whose connection to contemporary issues on which you have strong feelings, even perspectives which have been in close agreement over a period of time dating from the founding of Jericho through the Napoleonic Wars, diverge sharply over the qualities and virtues of individuals in the late 19th century, much less the 20th.