Mike,
Quite frankly... the materials that ATP sends you in advance should be more than sufficient.
Studying the Gleim books will help you prepare for the written tests and it's definately within your best interest to knock out as many of those as you can prior to day one of the program.
I'd study the Gleim intrument book and when you feel prepared to take that test... take the Instrument Instructor test immediately after (like after a 10 minute break). Both of those tests come from the same study bank so that makes it real easy. About a weeks solid in the Gleim followed by a few practice tests should be sufficient to get yoruself a 90+ on both of those tests.
The Commercial test will take about another week to prepare for. The FOI (Fundamentals of Instructing) is the easiest of them all and really only takes a short weekend to study before you can make a high score on that test.
As you are probably aware of by now... ATP is an immersion program that can get stressful at times. That's why I always recommend giving yourself a break during the program by knocking out these tests prior to it. That relieves a lot of stress and will definately open up a lot of free time for you during the progam.
You don't want to be a few days away from a checkride and having to study for a written, an oral, and a practical test all at the same time. So... while other's will be doing that, you'll be kicking back at the pool enjoying a cool beverage of your choice while perusing your oral study guide from time to time.
Bob