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.
Free Video Explanations: OFFICIAL GUIDE FOR GMAT REVIEW 2015.