I have not taken the PGRE anytime before, so cannot say which book has practice problems that reflect the level of difficulty in PGRE.
Conquering the Physics GRE is a book that explains exactly the situations what you might encounter in the test. Many of my friends have recommended me that book
The book focuses specifically on the TYPE OF PROBLEMS that the test will throw at you.
Mind you, it has 4 to 7 questions at max for each topic, which is not enough practice for the test, especially not, if you are from a non-Physics background.
I have another book: Sterling Physics GRE practice questions that has only practice questions and their solutions (60 of each topic). It has 12 practice test papers.
I use Halliday-Resnick as a book for re-introducing me the Physics topics that I had studied in my 11th and 12th class (I also bought my 11th and 12th Class textbooks for simpler problems).
As far as the study group goes, I am new to it and actually haven't done this before. Lets start to plan. Bharat1234 (in this thread ) also is preparing besides us. Although, both of you are planning on taking the test this September, but I will be giving the one in October.
All the best