What the title says.
I'm particularly interested in Young Kingdoms setting books/gazetteers (no particular region in mind) and adventure modules.
I own MRQII Elric which has a bitchin', comprehensive, but not very detailed gazetteer, and MRQII Cults of the Young Kingdoms.
Bronze Grimoire adds some fun magic stuff into the mix. The magic it introduces is not very "canonical" but it does add even more sword & sorcery crunch.
Well, there are a few region modules for older editions, such as The Northern Continent and The Unknown East, but I'm not qualified to comment as I haven't read my copies. I do recall reading very positive comments re: Sea Kings if the Purple Towns, comparing it to Griffin Mountain for RQ. In fact it may have been Balbinus/D-503 who said that.
I can say that although Sorcerers of Pan Tang is often praised, I didn't care for it, mainly because the matinee adventure is very linear, scene-ified, and full of illusionism that covertly reduces the impact of anything the PCs do. Also, I'm not so interested in the Elric universe per se, and I don't recall much that was stealable or inspirational.
It's not directly related to Elric, but the best supplement for Elric/Stormbringer I've ever seen is Corum, published by Darcsyde. It's awesome!
(http://www.darcsyde.org/Corum/images/Corum200.jpg)
I see you already own MRQ2 Cults of the Young Kingdoms. I like the way they approach Young Kingdoms religions a lot. When I was reading the cults description, often I thought to myself "these guys do get it".
The Unknown East contains descriptions of the kingdoms of the East, like Phum, you might find it interesting. Besides, it has a freeform magic system.
I really liked a lot of the Elric! supplements, particularly Atlas of Young Kingdoms, The Unknown East, Bronze Grimoire and Melnibone. Of the earlier supplments Rogue Mistress had some interesting bits (particularly the ship) and the Sea Kings of the Purple Towns.
I always thought that the location sourcebooks and the rules companions were better written than the traditional scenario books. This is probably because my Stormbringer group didn't need much help finding stuff to do in the Young Kingdoms. A lot of the time I was just holding my breath and keeping pace with their misadventures.
Another vote for Corum. Really well written, and the magic stuff is worth having for any SB-based game even if you don't use the Corum setting material.
KoOS