Currently the best study materials are the notes from your 100 hour basic course if taught by a certified teaching diplomate.
The links listed below are the reference materials for all questions on the Diplomate exam
Compiled Notes on Clinical Nutrition Products - Walter Schmitt - CURRENTLY OUT OF PRINT 2016 WAITING FOR UPDATE ON NEW VERSION
Some questions on the exam come for two sources that are no longer available to purchase, and the board is currently trying to rectify this situation:
George Goodheart Research Tapes - Currently unavailable to purchase 2016
George Goodheart Yearly Research Notes - Currently unavailable to purchase 2016