Only thing absolutely "required" is the PH since its pretty easy to use monsters from any D&D monster book you already have.
Aside from that? Anything you read and think sounds cool, it likely is. I really like Monsters and Treasures of Aihrde. Of Gods and Monsters has some issues with some items, and powers, but by an large I have really enjoyed it as a deities book. Despite owning many others, including Green Ronins incredible Book of the Righteous as well as Harn's awesome gods material.
I also own all of the modules, and ran all but like 4 of them. I was able to use them to have very good and memorable games, so you should be able to as well.
Only thing I have personally made no use of is the book on arms and armor. I forget what it is called, but I read through it and simply have had no use for it. You might, I don't know, I can only tell you I did not.
Lots of people seem to love the Castles Keepers Guide. I have used very little, but that is because I like my C&C the way it is. Since your new you may love more of it. To be clear, I in no way regret buying it. I am simply using very little of it.
