One would hope that when an organisation like Sun specifies a standard as important as a transaction api they might take the time to ensure they do a reasonable effort -- unfortunately you would be wrong. The JTA specification is freely available from Sun, however the document is extremely vague and requires the reader to infer the underlying transaction state machine - with the unsurprising result that the corner cases are left completely unspecified. So having waded through this morass myself, I include here my advice on what you need to do to understand JTA, in the hope that anyone else coming to this standard won't have to waste as much time as I have understanding it.
Read the JTA spec last. JTA is a little more than a thin wrapper around the Object Transaction Service 1.1 published by the OMG. This is one of the CORBA standards published in the late 90's and early 2000's - and like most of the early CORBA specs, it is well written; easy to read; and pretty complete. Unfortunately it too leaves the underlying state-machine and some of the corner-cases (especially those to do with recovery) underspecified. As a result I recommend printing this out and referring to it as an adjunct to the JTA spec.
Fortunately the CORBA-OTS is itself an object-oriented wrapper around another spec, the Distributed Transaction Processing: The XA Specification spec' published by X/Open (now The Open Group). There in Chapter 2 you will find most of the definitions missing from the other two specs; and in Chapter 6 the state-tables that provide definitive semantics for the various operations you will need to implement. You will also find a reference to another related X/Open specification - Distributed Transaction Processing: Reference Model - which contains all the remaining definitions and assumptions missing from the the JTA/OTS specs.
So if you do need to implement a JTA interface I strongly recommend you start with the X/Open reference model; then read the X/Open XA Spec; and only then read the JTA specification alongside the OTS spec for elaboration.