Thief 2: The Metal Age is a steampunk themed stealth game that's a lot like the original, only more refined.
You should start by playing Thief Gold because Thief 2 was made back when video games were more niche so developers expected people to play the original before buying the sequel, as such, the title features no tutorial and just throws you right into the first level.
The mechanics are very similar to the first game: monitoring light and sound is crucial for remaining undetected, stolen valuables are your main source of income, consumable items and ammo can be bought before most missions, and expert mode is the intended way to play because higher difficulty means more side objectives. Also, the game is incredibly atmospheric and immersive, as one would expect from Thief.
What differs this time around are the new stages, which feel a lot more focused on stealth this time around, enemies too. As an example, several levels in the original contained zombies that could only be permanently killed with holy water or explosives, whereas they are all but gone from Thief 2. There are also several minor quality of life improvements.
Combat, adventure and horror oriented missions are absent from the title and objectives such as "collect X gold" are equally scarce, giving the experience a more refined and focused feel. You can tell the developers had a more clear idea about what they wanted to do this time around whereas the first game was more experimental.
To conclude, I believe Thief 2 is an excellent entry to the stealth genre and I would have no issues recommending it to anyone. It's as good or better than Thief 1, but that's rather subjective and doesn't really matter because both games are great anyways.