Ah, sorry - I was thinking obviously more about at-table resources moreso than prep.
I like the original FR grey box set, but a lot of people seem to like the 3E book better. Both are excellent templates for setting up a vibrant campaign world, and for me the 1st ed is a more enjoyable read.
For 5E, I really like the Gamemastery series of books (Game Master's Book of...); they contain a lot of tables to build things on the fly and some of the later books in the series have excellent drop-in locations, NPCs and villains that are well-fleshed out.
I'm also a sucker for monster books, and the Monster Manual Expanded series is my go-to book (or at least my wife's go-to

). Lots of pre-5E monsters and advanced versions of existing monsters and very professionally put together.
A lot of my adventure design roots came from the old 1E & B/X adventures and disassembling those to put together my own stuff. However, I greatly enjoyed the Reincarnated series, especially "Into the Borderlands", "Isle of Dread" and "The Lost City" as not only did they reproduce the originals and convert these to 5E, but they got the authors to go into the design aspects of these adventures so you got insights into why they did what they did and point out some of the more interesting aspects of these adventures. Out of the three mentioned above I find I like "The Lost City" the most, because it isn't just a run-of-the-mill dungeon crawl, it's an investigation into the collapse of a whole society and interaction (and possibly rebuilding) while dealing with a powerful (but not world-ending) enemy force/organization.