As slave stories go, this one is, in my view, without peer. Northup's s captivating tale -- which has gained attention because of the movie that shares the book's title -- is told in exacting detail with an easy prose. He sets the stage masterfully, describing people and places before proceeding into the narrative. Unlike works of fiction, this book is so compelling because, by all accounts, it is true. There is no polemical axe to grind, as with Uncle Tom (a novel at one point wryly referenced by Northup). Here you see both the brutality of slavery and the moments of kindness by slaves and even some slave owners. Solomon tells the story with clarity and intelligence.
The free versions on other sites I found were pretty poorly formatted, so spending a dollar for a polished version on Amazon is worthwhile, but this one is not the best of them. Granted, the book is formatted adequately, and any typographical errors in this version seem to be simple reproductions of the original.
However, the supporting material is a letdown. I read the version that includes the introduction by novelist Dolen Perkins-Valdez. That introduction is borderline insulting, as it makes only a weak attempt to separate accounts with fictional elements like Roots from an authentic account like this one. Worse still, Perkins-Valdez can't resist indulging in repeatedly referencing her own recently released slave novel, even going so far as to quote herself. There are almost no historical elements to this version beyond the main book -- no mention of Northup after the book, no mention of he writer who helped him pen the book, nothing. There is more information on the writer of the introduction than there is the author. One other oddity worth mentioning: the original book's preface -- the one done by the man who helped Northup write the book -- has been curiously excised from this version too. That makes this version something less than complete.
For those looking for a better version, you might consider Twelve Years a Slave - Enhanced Edition by Dr. Sue Eakin Based on a Lifetime Project. New Info, Images, Maps, which contains a robust amount of supporting material and, better still, is right now the same cost as this version.