Other future histories may span more centuries or better illuminate the highlights of several decades, but until a rival is created with more historical depth and attention to detail, H. Beam Piper's Terro-Human Future History will stand as the Bayeux Tapestry of science fiction histories.
In many ways—certainly during his lifetime—Piper was the most underrated of the John W. Campbell's "Astounding" writers. He was probably also the most Campbellian; his self-reliant man is almost a mirror image of Campbell's "Citizen."
Piper died a bitter man, a failure in his own mind; shortly before his death he believed he could no longer earn a living as a writer without charity from his friends or the state.
Now he's the cornerstone of Ace Books. Had he lived long enough to finish another half dozen books, he would have been among the sf greats of the sixties....
But maybe he does know, after all. Jerry Pournelle, who was very much influenced by Piper and in many ways considers himself Beam's spiritual descendant—and incidently was John W. Campbell's last major discovery—has said that sometimes, when he's gotten down a particularly good line, he can hear the "old man" chuckle and whisper, atta boy.