Hi there,
The Foundations of Math book is designed to prepare students to take our 9-week prep course. If you feel okay about the material in the book, the next step would be to start learning strategies for more realistic GMAT problems. You could use our books (most people start with Book 1--Fractions, Decimals, & Percents) or take any prep course. However, I'd recommend that you continue to work with the Foundations drills throughout your preparation. Use them as a 15-minute warmup at each study session. If your basic skills are still sharp, it will be a lot easier to work through real GMAT quant questions with the right level of speed and accuracy.