The EJB spec is really important for the exam, some of the material in the spec is not covered well in any book. Not all of the spec is relevant though, since some of the spec is for container vendors only, so it is worth checking the exam objectives too. You would have to be brave to only study the spec I think.
The books Mastering EJB by Ed Roman and the Enterprise JavaBeans by Monson-Haefel are worth studying, as they are easier to read than the spec, and will help in understanding. The Monson-Haefel books is more in-depth in some areas, and is especially useful for container-managed relationships and EJB QL. (It does have a large number of typos to watch out for though.) And then there is the Sun J2EE tutorial, which does a very good job of explaining EJB concepts.
Kathy Sierra's new EJB cert prep book looks like it will be extremely helpful, so it is worth downloading the preview chapters as mentioned in other posts.