many inexpensive books on pd at amazon.com and elsewhere.
"THE PARKINSON'S DISEASE TREATMENT BOOK" by J.ERIC ASHLOG is good.
i'd read the sections covering your father's pd stage which most parkinson's patients eventually face. it will definitely help you discuss his options with a neurologist.

Deep Brain Stimulation - DBS - was developed to help treat the wearing off / dyskinesia problem with advanced parkinson's when all drug options were no longer effective enough.