Our local flight school had a dedicated AGI/IGI for a while. Lots of CFIs don”t like doing ground instruction and, if they are building hours, every hour on the ground is an hour they are not building time. I like teaching and so I just took and passed all three tests and need to fly over to a FSDO to get my certificate.
For the Fundamentals of Instruction Knowledge Test, study the Gleim book. If you have time, and if you want to be a CFI, at some point you will need to read FAA-H-8083-9A Aviation Instructor's Handbook but you don’t need to read it to pass the exam. A lot of the information in the book is wrong, so I had a hard time memorizing the wrong answer to questions. You probably won’t have to worry about that.
For the IGI and AGI it probably helps to read FAA books or whatever you prefer for getting your aviation knowledge. And you definitely need to study FAR 61 and 91 and know the VFR and IFR charts. I used the Gleim books Flight/Ground Instructor and Instrument Pilot and there were only a few questions that were completely new to me.
I’d say that about half of the questions are really stupid. e.g.
The force acting opposite of lift is:
Another quarter were interpolation/math questions that you will never ever need while flying. The study guides do a good job preparing you for them. The trick is to read the question really carefully. e.g. Are they asking you to look on the chart for landing distance or do you need to do another calculation to get ground roll.
There are a bunch of E6B questions that you just need to be really careful in your arithmetic and reading the question.
There are a few that I really had no idea what the question was, let alone what the right answer was. I got about half of those right. There were also a couple on each test where none of the answers was the correct answer.
I took the Commercial and CFI since they are good for two years and I won’t have to study them again if I decide to get a CFI.
My results: FOI 84%, AGI 88%, IGI 90%, CFI 87%, COM 92%.
I studied for a month, which was a bit of overkill.
Sporty’s and King Schools let you take practice tests for free. I put a lot of my notes up on my website so you can save time when studying by printing and memorizing them.