The Official Guide to the GMAT is the single most important book for preparing for the exam, at least in the initial stages.
There is a massive overlap between the 13th and the 12th edition. If I had no money, I would just use the 12th Edition. However, if you can spare $25, then this is what you obtain:
1) An entire section on the Integrated Reasoning. This is absent in the 12th Edition. This includes an online access to 50 practice questions that is valid for 6 months.
2) A total of 160 new practice questions: 80 in quant and 80 in verbal. Of course, they are also dropping 160 questions that are present in the 12th edition and not in the 13th edition.
New!2016 OFFICIAL GUIDE FOR GMAT REVIEW: Free Video Explanations.