someone else hints at this but one of the easiest Messiaen pieces to understand is his "Quartet for the end of time", written while he was a prisoner of the germans during WW2. The wikipedia article for the piece is actually pretty good "liner notes" if you want a guide to each movement.

One other thing about whatever you want to call the contemporary continuation of classical music - you're right that there's often a lot going on. I've heard it described as "fragile" music: in contrast to pop songs that still work if you're on a train, if they're interrupted by an announcement, if you only listen to the second half, etc., a lot of this music is written to be listened to by someone who is relatively calm and undistracted, in a quiet environment. And honestly some of it has additional expectations of its audience, like a better than average ability to remember melodic snippets and recognize their transformations, or an ability to track "advanced" harmonies and key changes. But imo the general rule "90% of everything is crap" applies in this genre as well, and most of the pieces that everyone will speak of in glowing terms have a way of 'working' for most listeners without demanding advanced education, study, or above average musical-listening abilities.