The Official Guide is made by the people who write the main test, not by ManhattanGMAT. It is definitely important to buy the Official Guide (the latest is the 12th edition) because it contains real past test questions - those are the best questions from which to study.
ManhattanGMAT's Strategy Guides teach the content and techniques for answering GMAT questions. For quant, there are 5 guides total (1 Number Properties, 2 Equations Inequalities and VICs, 3 Word Translations, 4 Fractions Decimals and Percents and 5 Geometry) and, together, they cover all of the quant material. There is also a Foundations of Math book for people who haven't taken a math class for a very long time or who are scoring below about the 50th percentile - if that's the case, then we recommend to our course students to start with Foundations of Math before taking our course (where we use the 5 regular books). Finally, there's also an Advanced Quant book that addresses problem solving and analysis techniques for those who are currently stuck around the 75th to 80th percentile and want to score in the 90+ percentile.