Our best investment was subscribing to TourGuideMike.com! You will need time to look through all the information the site has because there's a LOT there! The really great thing is that Mike updates his information all the time so you'll always have the very latest info. We usually walked right on to attractions while there in early July for our first WDW trip and loved it! When we did stand in line it was usually only for 5-10 minutes. Plus, TGM has a money back guarantee if you find you don't like it for any reason.
I bought a Passporter and love the maps but I wouldn't buy it again since for me, it didn't really add to what I wanted/needed to know and I don't like taking bulky things on trips.
Also like to read the OG from the library but the information in guide books are always going to be old (unlike TourGuideMike) since it take a lot of time for writers to do their research, write, publish, etc., by then at least 6 months has gone by and things change all the time.
In any case, good luck and have a great time!