Ironsworn: Delve is a 240 page expansion to the already beefy Ironsworn core. It has the same extremely solid level of quality, and it adds rules orbiting around exploring dangerous adventuring sites. Furthermore, Delve also throws in a bunch of sample dungeons, monsters, and anomalies (basically locations that are also monsters,) plus a *lot* of new character build options, including Assets that drive gameplay themed around money and occult artifacts. Content-wise, pretty much everything in core is enhanced or optionally altered here.
Core Ironsworn primarily involves travel and overworld encounters, so Delve surprised me a bit with the amount of energy it devotes to making a separate game out of dungeon/location-diving. There's a set of Moves and mechanics specifically to handle adventuring sites, and they're comprehensive---although they feel really similar to the way Ironsworn handles combat and travel, so you don't really have to learn anything radically new to use them.
Delve also comes with a flexible, robust dungeon-generator. It's done primarily through keywords, which you can randomize if you like, and which are general enough to cover most possible options for spooky places you might want to set foot in. The generator is strongly tailored to Ironsworn, so you won't get tactical grids out of it, but if you want to just yank out the generator and use it elsewhere, it *is* very good for quickly coming up with overall pictures of dungeons and their inhabitants.
Provided you *are* intending to use Delve with Ironsworn, it contains lots of non-dungeon elements as well. There are rules suggestions for how to link the site-exploration mechanics to quests or travel, and for how to recalibrate the overall game difficulty or streamline play, and for how to add maps, and for how to add exp gain from failed rolls. Like Ironsworn, Delve is built to be hackable, and it comes with frequent recommendations for how to do that.
Overall, if you like Ironsworn, it's absolutely worth supporting Delve. Delve doesn't radically change the game's contents, but it provides a wealth of support and resources and fleshes the game out even further.
And if you haven't checked out Ironsworn yet, do that. It's huge, it's free, and it's very well made.