I studied for the IRA, FII, and IGI exams all at once, using only Sheppard's IRA study guide. Got three scores all within a point of each other (around 92%, thanks to SA's great study software). No need for separate study guides.
FOLLOW their instructions on how to study! I managed to pass the ATP written a couple days before the 31 July deadline, only due to Sheppard and their study strategy.
People get the ground instructor ratings for a few reasons: if you lose your medical, you can stay involved as an instructor, and you have to have a ground instructor rating to be eligible for Gold Seal CFI.