Hi Tartan and welcome to MR!
Since I haven't got a symbian os device or use ereader myself i can't check but .kz seems to be some kind of metadatafile ereader uses. Have a look at this thread here
The actual text could be in files with a different format / extension name. I guess one should be able to handle those with calibre.
The .jar - files you are talking about run with a Java-engine. I don't know if there is a reading software for Symbian OS v9.4 (which your phone runs) for those.
By the way: You could try if the old Mobipocket Reader for Symbian
runs on your phone. In that case you should be able to read all those .prc and .mobi files floating around here and elsewhere. But be aware that this is old software and mobipocket essentially is on it's way out of the market. Edit: Also have a look over here (for the N97).