If you want it purely as an ereader and you will be reading a lot then it would be advisable to get an Amazon Kindle. As Galaxiom said there is a special type of screen for ereaders which is more gentle on your eye causing far less strain. In the past Sony ereaders have had large failure rates (search google to see how many complaints there are) but the Kindle has a good reputation and is a reasonable price. I am not sure what other alternatives there are to the Kindle.
If you have more money and would like a multifunctional device then perhaps consider an iPad or await the iPad 2 which will have a much higher screen resolution. The iPad may strain your eyes if you read for long periods due to it using normal screen technology but it can surf the internet, use apps etc.
I personally feel a phone screen will be too small to read, even if you can zoom in. I have the iBooks app on my iPhone 4 but I wouldn't want to read a novel using it.
The best thing is to try the devices out before you buy, I am not sure where you could demo a Kindle or other eReader but you could try an iPad inside any Apple store.