Gee whiz, I bought that set at my library book sale a couple of weeks ago for $6.00. I got it to go with the 1992 Encyclopedia Brittanica that I bought for $20 (they're both in blue leatherette). I can hardly believe this set is worth so much... did you check eBay? The book dealers passed it by and it sat on the table until the very end of the sale. I bought it because it was in such great shape, matched the encyclopedia, my son likes history

and it was going to be given away anyway. I'm a sucker for a book and volumes of books are even better.
It is the story of America, told by the people who lived during the time. The first chapter, for example, is a report written by Columbus, the next is by somebody named Sparke who is describing the wonders of Florida. Each chapter is a verbatim (but not facsimile) reproduction of what the editors considered to be a "telling" moment (ie. a report, letter, or document) in the history of the continent. A little dry for my taste, but I think my son will enjoy it.
The entries start with a brief statement of the significance of the chapter and include the original source for reference. The entries seem to be rather short. There are quite a few illustrations, all in b&w. This is a 20 volume set from 1976... according to abebooks, I'm missing two soft-cover volumes: one being the introduction to the series, the other, an index. Luckily, the annals are in chronological order.
What else do you want to know?