The one says Practice Exams, and therefore I assume contains practice exams rather than text. The other is the text that explains the subject that the exams cover. In the previous book on this subject the Programmer, Developer and Mock exam material was all in one book. Looks like they have separated the Explanatory text and mock exams into two books and put the developer material on CD ROM.
(My comments are entirely as an observer, I have no "inside knowledge" on this)...
The 'study guide' is almost done - we're 1/2 way thru the last chapter!
The 'study guide' teaches all of the topics for the Tiger SCJP exam, and it has lots of mock exam questions, there are many mocks in the book, more mocks on the CD that comes with the book, and then more mocks on a website that you can access when you buy the book. (We're shooting for a total of 280 questions.)
Because the new exam is so big we won't have room for SCJD material that was in the previous book - I think we're going to put those chapters on the CD.
The other ('practice exams') book is still in the planning stages, I'm not even sure why they put it up on Amazon We're thinking about doing a book that is just mocks and exercises, but we haven't even decided for sure when and how - don't wait for that one!