I’d say his books--at least those two, because those are the only I’ve read--are targeted more towards teens. They can be read by both, but they’re written from the perspectives of teenagers. And noooo, they’re not like Nicolas Sparks books at all, haha. The writing style is definitely different and the characters in Looking for Alaska especially have, well, unique and bold personalities. Both Looking for Alaska and The Fault in Our Stars do have sad endings (sorry if you didn’t know already) but they’re not as...should I say, serious?....as Nicolas Sparks books. John Green’s books have a lot of humor in them. They do have a little romance (at least The Fault in Our Stars does) but definitely not as much as Nicolas Sparks’ books.
By the way, The Fault in Our Stars is written from the eyes of a girl cancer victim and Looking for Alaska is from the point of view of your typical teenage boy. Just saying.
And maybe I’m just biased, but I love John Green’s books. If you’re looking for both humor and tragedy and doses of reality, read his books. His books definitely make you...feel something.