"Boxed set" is a bit generous; Al-Qadim, Dark Sun and Birthright all had sourcebooks/modules that were put into a slim box set format. You probably know of them, but they aren't "thick" enough to really register as boxed set. For Birthright it was:
- Cities of the Sun
- Havens of the Great Bay
- Tribes of the Heartless Waste
- Rjurik Highlands
and the campaign set.
For Al-Qadim, you had the likes of:
Adventures
- Golden Voyages
- Assassin Mountain
- A Dozen and One Adventures
- Secrets of the Lamp
Region sets
- Ruined Kingdoms
- Corsairs of the Great Sea
- Caravans
- Cities of Bone
The real "boxed sets" for that setting were "Land of Fate" and "Cities of Delight".
Dark Sun had the folio adventure sets, which were sort of like boxed set adventures. It too had several boxed sets - "City by the Silt Sea" and "Ivory Triangle" was a full-sized one, and seem to recall there were other slim sets (all my stuff is in storage, so going by memory) such as "Windriders of the Jagged Cliffs", and "Mindlords of the Last Sea".
2E got super-happy with the boxed sets, because they appeared to sell better than softbound books. However, I seem to recall articles/discussions that they generally ended up costing most than they made back*. I also imagine they had a lot of returns as back in the day when I would go to the bookstore I'd find those sets had been opened and rifled through - and there's few people I know who'd buy an open box for fear something would be missing.
* Even worse were the "faux leather" cover books like Complete Shi'ar, the other Complete books and the Historical References, whose gold lettering and cover were exceedingly expensive for the time.
