I've been happy with Oahu Revealed. I've rounded that out with this board, also the CruiseCritic forums, Google Maps (to create my own map of things I want to see/do and just to get a better sense for how the island is laid out), Yelp (to get more info on restaurants). I use Google Flights to search out all of the flight options (learned quickly that in our case, expanding to a nearby international airport gave us a much cheaper option), and Kayak and TripAdvisor to get additional input about which hotel we'd want on Waikiki for a split stay. As an over-planner with our WDW trips these all helped a lot.