Hi artjak yes the leaves are edible - they're really good for flavouring stocks and soups - their flavour is pretty strong, stronger than ordinary celery, so you don't need may of them to add flavour.
Dove, I've been growing celery for the last couple of years and found the flavour of home grown almost eye-wateringly strong, but v. useful in cooking. I just don't have the space for both veg so growing celeriac gives me, I hope, the best of both worlds.
I still have about a dozen celeriac growing, artjak. They do need a long season - with the late start last year the roots weren't big enough to pull until Christmas! I'm going to start them off a bit earlier this year, weather permitting.. I'll pull one or two this weekend and post photo's. They are lovely roasted but an absolute pain to prepare - the useful part of the root is completely covered in small roots which, in my clay soil, really hold on to the soil so cleaning is more like slicing through the roots/mud until the 'outer heart' is reached. Not a lot was left after that in my case! They seemed to grow completely pest-free though!