There are so many good things to say about Twelve Years a Slave (book version) I'm going to have to just give a brief summary.
For a memoir, it reads as well as a novel, at least a 19th century one (and better than many 19th century ones). Once I started reading it I had a hard time putting it down... so beyond any other consideration, it's a good book.
As a historical document for 21st century readers to use to learn about slavery in the American South, it has the advantage that it was written for an audience who were also foreign to that situation. It explains everything a reader could need to have explained to them in order to understand the system of slavery. As a historical document relating to how the battle for abolition was fought it's also an important work, but a bit less clear-cut and easy to assess without more background knowledge, I think.
In comparison to the film I think it has the enormous advantage of being a first person narrative, in which Solomon Northup's thoughts, judgments, analyses, plans and feelings are given more importance than the appearance of what was happening to him. It's also able to convey a lot more information than a film can realistically get across - the biographies of numerous slaves and slave owners, their networks of relationships, the organisation of work throughout the year, economic details... Sometimes the great thing about the written word is its ability to tell rather than show.