The Official Guide (OG) is a great resource for practice questions, but it is not necessarily the best resource for solutions. For example, one of the great things about GMAT math questions is that they can typically be solved using at least 2 different approaches (one fast approach, and one or more long approaches). The OG often provides the longer (often algebraic) solution. For example, you seldom see the OG use estimation to solve a question, or plug in the answer choices, or rely on some number sense, etc...

Similarly, the OG doesn't spend much time explaining the rules of grammar (for SC questions).

Given all of this, you need at least one more GMAT resource that will provide the necessary content knowledge AND GMAT-specific strategies for solving questions quickly and accurately.

You may also want to sign up for BTG's free 60-Day Study Guide: ttp://www.beatthegmat.com/mba/gmat-guide
It will help keep you on track. h

I hope that helps.

Cheers,
Brent