All of the humorous novels that I have ever read have been teenage ones. My English teacher once told me that it was pretty difficult to write humour, and I believed him until I grew older and began to write fanfiction, which had me and everybody who read it in absolute stitches. The book that had me laughing the most was a book called Angus, Thongs, and Snogging, I have forgotten who wrote it because I read this book back in primary school.
It is difficult to make humor; at least compared with drama. Anything can be dramatic and people will comprehend it as such easily. You can't really miss drama. At worst, if you're a sucky writer, it will seem a bit superficial, but it will still carry the story to interesting places. With humor, everyone has a different taste and different ideas of what is funny; context and timing is very important and can be screwed up very easily. You can go into the 'too much' or the 'too little' or the 'wtf is happening here' very fast. There is nothing more boring than a humorous novel that fails to bring humor.
I laughed out loud at many fics; they fell under my domain and they played with continuity/inside jokes and hilarity pretty good.
I did not laugh at many books that were supposed to be funny.
I did laugh a lot at Sophie Kinsella's book about Emma's secrets and all. Later on, it was not so funny anymore, though still humorous. Funny books are even harder in the regard that you expect the joke and punchline the next time, so it has to be skillfully crafted to deserve a reread (if you are into rereads, that is)