We did invest heavily in Fourth. In a strange attempt to collect them all. I have no regrets. Well, very few. The Player’s Strategy Guide comes to mind. Luckily, for now, there’s not a lot of product out in way of Fifth.
I’ve got the Player’s Handbook and am looking to get the Monster Manual that recently dropped. As far as DM’ing goes, there are some monsters in the PHB and a few you can find here, to help round things out while you wait for your coin purse to refill. The book I’m least looking forward to is the Dungeon Master’s Guide.
DnD, and DM’ing itself, is a game that can’t be taught but must be learned. You need to know the core mechanics of the game, but you have to be willing to put in a lot yourself, which is something the books don’t – and can’t -provide. The thing that makes me least excited about he DMG is that I have my DMG upstairs, in the “old editions” section of my personal library. I have both the DMG and DMG2 for Fourth Edition, but over the last few years playing that edition, I’ve consulted my DMG from Third.
So while I’m least excited about the DMG, I’d recommend to anyone new out there, seeking a level of mastery over dungeons, to pick it up. That book opened my mind up to new ways of thinking (about the game and the world itself!). That book carved out whole sections of my brainscape that could have been used for something much more useful and left a tool kit. I’ve been filling that space with whole worlds in the interim.
Your first Dungeon Master’s Guide is an eldritch tool, through which you acquire the skills of a DM. Fourth was, and Fifth will be useless for me in that regard. But for you, dear reader, it may unleash such power that you may be reluctant to ever part with it. Good luck.