House of Earth and Blood → House of Sky and Breath → House of Flame and Shadow. The series must be read in order — each book ends on major revelations that the next picks up.
Yes. As of House of Flame and Shadow, the Crescent City universe directly connects to both the ACOTAR and Throne of Glass/Crescent City universes. House of Flame and Shadow contains spoilers for the ACOTAR series, so read ACOTAR first.
Adult. Crescent City is Sarah J. Maas's most explicitly adult series — grittier, more violent, and more sexually explicit than ACOTAR. Not suitable for younger YA readers.