The D&D community is abuzz with excitement about the new Waterdeep module, Dragon Heist. Aside from maybe Curse of Strahd, there seems to be more chatter and anticipation surrounding this release than any other so far. There is already a steady stream of fan content coming out on the DM Guild site, but as Waterdeep is probably the most famous D&D city of all time (other than maybe the City of Greyhawk), DMs can also benefit from the classic-era products that came before. Here are a few to check out (and some to avoid).
CITY SYSTEM: HIGH USABILITY
Known for its massive map collection of the city, the real strength of this set comes from its booklet. Filled with random tables and useful info to expand your campaign, this is in my opinion the #1 classic-era product for Dragon Heist DMs.
VOLO'S GUIDE TO WATERDEEP: HIGH USABILITY
The creative travel-guide presentation of this book serves it well, effectively humanizing (demihumanizing?) the city with flavorful entries. Compatibility with the City System map keys is a huge plus. Part of what makes this and the City System box so useful is that most of the material doesn't rely on then-current events.
FR1 WATERDEEP AND THE NORTH: MODERATE USABILITY
This was the Waterdeep bible for a while, and it is a good book. Much of it could find some use in your game, but a good portion is focused on then-current events. I am no Realms expert but I am pretty sure they have been like exploded and put back together a lot of times since this came out, and a lot of the NPCs have been dead and buried for years.
THE RUINS OF UNDERMOUNTAIN: LOW USABILITY
As I expect most Dragon Heist DMs will segue into the upcoming Dungeon of the Mad Mage, you can skip the original Undermountain. There is scant Waterdeep or Yawning Portal info to be found here.
FRE3 WATERDEEP: COMPLETELY USELESS
Don't be fooled by the title. This is one of the worst modules TSR ever released. This is a novel tie-in that makes the Dragonlance adventures look like sandboxes. The party doesn't even get railroaded to Waterdeep until towards the end. A couple of generic floor plans are all you might find useful in this turd.
Note: I don't own the City of Splendors box set, so I didn't feel comfortable recommending it. From what I understand, it reprints a lot of FR1 and the City System, so I never felt the need to seek a copy out. That said, it could be another good option, just keep in mind it might overlap a lot with other products.