Well you're on the right track with the Seminole supplement and POH. The more you can understand everything in the supplement the better.
As far as anything else, well it depends on how far advanced into your training you are and what writtens you have done. Immediately following your Multi add on you will be starting the instrument portion. So the Jepp Instrument/Commercial text is an excellent source to start reading along with the Instrument Flying Handbook. Study the Gleim instrument book as well for the written.
Concerning the attrition rate and the grading of your performance, there is no set flight check or letter grade for each phase. Your instructor is the primary evaluator of your performance and whether or not you will make it through the program. I can tell you this - if you have a very positive attitude, a willingness to learn and study hard, you will have no problem what so ever with the program. Ask questions anytime you don't understand something and make sure you have a good relationship with your CFI. His job is to train you, your job is to learn. So develop a good line of communication between each other. The attrition rate varies and is dependant on how well you retain information and perform on checkrides. There is no exact figure to my knowledge, but most students are successful with the right attitude.
You won't get kicked out of ATP for failing a checkride, but the Instrument checkride is the single most important ride of them all. Fail that checkride, and your performance will be evaluated by ATP admin and decision made as to whether or not you should continue. The main reason for this is the insurance policy reguarding you flying with another student on cross countries. You must be very sharp on the instruments and very proficient, as you will be flying cross countries with another instrument student and rest assured you will be flying through IMC and shooting approaches. Students have failed the IR checkride for little things and still continued. But busting for something big, like going below MDA of DA when unauthorized to do so, may bring up some serious safety issues as far as your cross countries are concerned.
BUT, do not worry about it. The instrument checkride is hard, no doubt. But study hard and take the right steps to prepare and you will do fine. Your instructor will prepare you for the instrument checkride and he will not send you if you are not ready.
Overall its not that difficult of a training program. Its very regimented, its very structured and requires a good effort on your part. Do that, and you'll be building time as CFI in no time!
