The Turbine Pilots Flight Manual was useful to me and so was Everything Explained for the Professional Pilot. If you are a mil helicopter pilot, spend some time reviewing the Jeppesen plates and how to brief one. I would be careful with the "study everything" concept as you might bite off too much and miss the basics like weather planning. My impulse is weather planning items are non negotiable.
Don't discount the HR stuff as less important. E.g. the infamous "what's one word that describes you". Answer it with your one word like "professionalism" or whatever but also have a scenario where you earned it. If you just say "professionalism" and move on, you missed an opportunity to show the interviewer your personality and to sell yourself as a professional.
Not sure if you are going for RTP, cadet or a first officer gig but I dramatically overprepared (studied for a RTP 30 question written tech as if it were a full on oral exam like a 1500 hr ATP candidate). Knowing I would catch the curve balls made me more comfortable and able to come off as "the kind of guy we want to fly a 4 day with" which seems to be the most crucial metric. That and weather planning.