All batteries have a life expectancy measured in charge cycles, so yes, continually recharging it will shorten the life of the battery in your Kindle - its a pretty high number though so even using it as you are it'll still last for a few years I'd think.
Even if you read a lot and have the light full the whole time you should be able to wait a few days to a week before it needs to be recharged, it might do no harm but there's no need to keep it topped up the way you re doing and it is probably better for the battery to let it drain (though not completely) between charges. (edit perhaps I'm wrong about this considering what HarrryT has posted?). Though you won't kill it if you use it as you are doing......
Last edited by soulfuldog; 01-12-2013 at 06:12 AM.