My apologies, I wasn't clear enough before. Also
libtcod, seeing as I forgot to give a link to the actual thing. libtcod (AKA TCOD) handles the display end of roguelikes, along with a few other (awesome) tools.
If you want an API with things like pre-defined creature or item classes, then I don't think you'll find a (good) one.
The majority of roguelike "engines", to use the vernacular, as far as I have seen, are for putting the @s and Es on the screen and moving them around. It's up to you to figure out when they're talking or fighting or trading or whatever - that's what game development is about. Doing stuff.
If you want to "piggyback" off someone else's code (which is not a bad thing unless it breaks copyright law or something), try looking for an existing open-source roguelike and using what they've written to make your own game, or mod. The same goes for existing engines.
I wouldn't recommend this approach, honestly, because you'll have to learn how the other devs do things which is sometimes harder than doing it yourself.
Really, how you choose to go about this depends on what you want to get out of it. Do you want to learn programming? Try making the game engine. The nitty-gritty code. It might not work the first few times, but you'll be a better programmer by the end of it.
Do you like game design? Planning mechanics and creatures, thinking of cool spells? Try modifying an existing game, or finding someone who is willing to do the coding for you. Working with another person is often a good idea, especially in game development.
If you want to "make a roguelike"... you're going to have to do the hard yards first. There's no "easy way" to make a good anything, other than hard work, which isn't easy at all.
you will have to get your hands dirty