[Rizla]

If the Nook St gives you eye-strain, then you'll get it with all e-readers. E-ink tech hasn't improved all that much since the Nook ST. Don't get a tablet. They are far worse for eye-strain. Do you use computers a lot? That can cause it (it does for me). I tend to borrow paper books from the library and get them cheap from abebooks these days.

For an e-reader, I'd suggest a Kobo Aura H20. The screen is a bit bigger and it has a good in-built light that will help the eye-strain a bit. E-ink works better with good lighting. Make sure you always read on e-ink with a very good light source.

Also the Aura H20 is pretty friendly when it comes to reading books from other sources. But be careful with it. There are reports of it being more fragile than a 6" screen.

You could get the Kobo Glo HD. It has all the benefits of the H2O but is only 6" and cheaper.

You could also get the new Nook and root it, but that's only if you're confident doing that kind of stuff.

I suppose you could get a Kindle and buy from other sources, convert the books using Calibre, then side-load them onto the Kindle. The Kindle's are getting increasingly restrictive, so that's something to be aware of when / if you buy books from Amazon, but there's no reason you couldn't side-load books onto the device bought from other sources. The hardware is good and if it breaks it will be the easiest to replace.

Saying that, the font options are nowhere near as good as the Kobo. The Kobo will allow you to make fonts bold, which I think is very important if you suffer eye-strain. You can alter fonts on the Kindle, but its not at all friendly.

Maybe the Kobo Glo HD is your best bet. Inexpensive with an excellent screen and excellent font options. But if it busts, it would be a hassle dealing with Kobo (via mail). Should be okay of you look after it though. But there is always a chance of a lemon like with any device.