Fundamentals of Instructing
When you place a deposit, ATP sends a box of books that includes the Jepp I/C manual, some FAA books, and all of the Gleim study guides. Also, it is my understanding that once you have placed a deposit, ATP will reimburse you for the cost of taking the writtens once you arrive for training. Note that the instrument and CFII are from the same pool of questions, and the commercial and CFI are similar in content. Fundamentals of instructing (FOI) is basic psychology and instructional methods. My point is that there are three main subject areas to cover, and I suggest studying for and taking related exams together to minimize workload and re-learning.
I'd advise taking the FOI as soon as possible-it's very easy and is great to get out of the way.