Dennett's basic thesis is that most of the fuss about free will has been caused by the summoning of bogeymen — non-existent and sometimes barely credible powers that are supposed to be able to interfere with our free will in a deterministic universe. The opening chapter, "Please Don't Feed the Bugbears", looks at some of these bogeymen, and discusses the more general use of "intuition pumps" (stories that appeal to our human level intuition to prejudice us for or against more technical ideas). The following chapters lay the groundwork for understanding different conceptions of free will: the second discusses "reason", the third "control" and "self control" and the fourth "self" and ideas of "self-made selves". These concepts are set within an evolutionary context.
The next chapter, "Acting Under the Idea of Freedom", looks at how we can continue deliberating while believing that the universe is deterministic. In particular it considers different definitions of "opportunity" and "avoidable", and how these things tie in with real life deliberations, motivations and expectations. The chapter "Could Have Done Otherwise" finally takes the lid completely off the metaphysical "can" of worms (Dennett is fond of the occasional pun). Retrospective desires to change the past, wanting to be able to make several incompatible choices at once, confusion about the difference between the actual and the possible, the role of chaos in physics — these are just a few of the things considered.
The final chapter looks at what Dennett considers the most important question — Why do we want free will anyway? Dennett thinks that the fears raised by hard determinists and incompatibilists are about kinds of free will which aren't really worth wanting anyway (when they are not simply self-contradictory). Before getting worked up considering the details of their arguments we should consider whether we really care about what is at stake.
For anyone concerned with arguments over "free will" Elbow Room will be essential reading. (It is also suitable as an introduction to the topic.) Elbow Room may also be of interest to a wider audience; even if you are uninterested in or bored by the metaphysical "Problem of Free Will", you may find Dennett's ideas on such topics as rationality, selfhood and personal identity thought-provoking. (If that is the case you should also have a look at his more recent Consciousness Explained.)