While waiting for my I-129F Petition for Jaycel to be approved, and bleeding over into waiting for the embassy process, I read the entire 2500+ page USCIS Policy Manual and the relevant parts of the FAM (Foreign Affairs Manual) so that I would understand the next phases and how USCIS / DOS approach the process in general. I know that's not everyone's cup of tea, but like @Crazy Cat said above, at least have a firm grip of what you'll need to do for the embassy stage and it's never too early to start studying the AOS process. You'll be surprised how fast the 90 days goes by once your fiancé(e) arrives.