Manhattan Quant Books Study Plan & Foundations of GMAT Math?

I would recommend you to finish all the solved examples at the end of each chapter first from the MGMAT rather than trying to solve all the related question from OG as well. Problem with latter is that you may be stuck with MGMAT and OG for a long time that way.

If you just solve the exercises from MGMAT, you will be done with the guides faster and it will be less boring. After you are all done with MGMAT theory and solve examples, try to solve related topics that's mentioned at the end of every guide. This way you will get a chance to revisit the topics you may have forgotten from your first reading of the MGMAT theory. Also, if you get bored of doing too many OG problems, you can take practice problems from other sources and try to solve them. After all, you are already done with your theory.

I feel MGMAT guides are pretty decent for startup. You may have to practice problems, read from other sources to better yourself in advance topics of inequality and permutation/combination/geometry.

As for other source, I feel GMAT Club math book does a pretty decent job in summarizing most of the formulas and technique:
gmat-math-book-87417.html

I remember taking a cursory look into "Foundations of GMAT Math". I didn't find it to be too valuable.

You can read this if you have ample time:
new-to-the-math-forum-please-read-this-first-77764.html