Not GP, but The Joke changed my conception of what a novel could be. It was the first Kundera I read, nothing after that came close, including ULOB. But The Joke was so good I had to read everything else he wrote just in case.

Maybe "risibles amours", but biased as it was my first book from him.

As novel, it is hard for me to split between immortality and the joke. The joke touched me more, immortality is more "serious".

But hey, I am French, so my immune system against this kind of thing is definitely weak. And I did read those as teenagers, and girls were definitely part of the appeal, at least originally