Recommended Readings

Must read

If you are seriously considering using an XA transaction manager in your application, it is highly recommended to read Mike Spille's XA Exposed I, II and III.
Those blog posts summarize the most important things one should know about XA and - cherry on the cake - are quite pleasant to read.

Specifications

The JTA specification is a good read to get in-depth details. The XA Specification should be read in parallel as the JTA spec implicitly depends on it.

Misc