Twelve years a slave the movie is based on Solomon Northup's autobiography, published in 1853. I've actually never seen anyone question the histocracy of the book, as it was written by a former slave about his times as a slave. If anything is off about it, those things are minor and based more on the issue of human memory. He was a real person, his story really did happen, and there's a good deal of evidence to back it up. There's even an updated version with background information, maps, etc that was put together by Sue Eakin and Joseph Longsdon, which included references for all the information they found about what happened.

The movie is mostly accurate to the book, although it has a few changes I can think of. If anyone else noticed any differences, feel free to point them out.

At one point in the movie Solomon is nearly hung by a slave handler, and is left half-hanging as a punishment until his master arrives to rescue him. In truth, his master had already sold him to the slave handler, and the only reason his former master saved him was because his slave handler had not yet paid off the debt owed, which meant he wasn't allowed to kill him until it had been paid off.

A lot of minor scenes were cut out simply for time such as one where, while being transported south, a sailor helps him write a letter to his friends in the north.

Michael Fassbender's character, Mr. Epps, is changed slightly for the movie. While he was pretty much just as much a monster between book and movie, in the book he seems very disillusioned and neurotic, as opposed to simply cruel.

There were a few other changes I can only half remember, so I'll leave it at that. For the most part, the story is almost entirely intact. This isn't a 'based on true events' where 90% of the story is different, but instead one where the only differences are fairly minor and subtle, chosen for time constraints more then anything.

Hopefully that answers your question.

In addition to what others have said: Patsey in the movie asks Solomon to kill her. But in reality, Mrs. Epps asked Solomon to kill Patsey.

There's nothing in the book abut his first master giving him the violin as a gift.

Solomon's family knew for almost the entire time he was gone that he had been kidnapped and enslaved--they didn't know where he was, though, and couldn't help.

The brutality was greater in the book than in the movie.

The movie leaves out the ways that the slaves were constantly under surveillance to keep them from escaping.

But these are fairly small bits. They really tried to keep close to the exact events, with the language almost verbatim.

One of the biggest changes is near the start. When he is on the boat no rape occurred and neither was a slave killed by a sailor instead he died of smallpox. You have to think that a slave is a very valuable piece of property and no poor sailor would ever kill something that cost 1000 dollars in 1840.