Sometimes youth books are not necessarily the easiest to read. My daughter could tell you...
Anyway, prefer general topics, to avoid too specific vocabulary (from this point of view, I do not think Harry Potter is a good idea). Although a little outdated, Jules Vernes is a good starting point. He has enchanted generations of french boys since decades.
You may also google for [please do not name pirate sites - moderator], but this is another story...
