Dear Alex, Program management methdology and the standard is still in novice stage (as compared to project or portfolio) and so the things are changing fast.. Further, very few people are taking the PgMP exam. Though there were books in the past on PgMP (by Phillips), becasue of the changes in the subject and standard, they are obselete.
So currently to prepare to PgMP, I would suggest to go with Dr. Ginger Levin Question book. (it is not similar to Rita book for PMP). This book present you a questionnaire, may be bit more than exam complexity.
I think you can use this book in an OPEN book mode with PgMP standard and ECO, which will be very effective. In case of PgMP, the standard is principle based standard and provide you high level info about the program management concepts. Whereas exam expect you to have a detailed program management skills ( scenario based questions). So the overall effort should be towards understanding the concepts and how they may translate into the scenarios or your work. This is bit of a challenge. This is where critical thinking helps.
I do have Online PgMP prep courses from time to time and will be happy to assist you in your journey. Saving Changes...