Buy a Kindle Paperwhite, it's only $119 and is the best reader at any price.
I use my iPad 4 & iPad mini for everything else.
Nothing beats a purpose built reader like the Paperwhite. Period.
For anyone that reads, I wholeheartedly agree that you should get a Paperwhite. I own an iPad mini and a Paperwhite, and while I read sometimes on my mini, I mostly prefer my Paperwhite, it's a great device.
What I wanted to add to the conversation was that there is a great site where you can get free eBooks in any number of formats:
Project Gutenberg. They take books no longer in copyright and make them available for download, it's legal and free. Amazon also has quite a few books that are free for download (and I don't think you need an actual Kindle, just the Kindle app on an iPad or on your iMac will suffice I think). If you like classics, those two are great places to find loads and loads of great free stuff. As an aside, if you like audiobooks,
LibriVox has audio versions of some of these same books you can get free in other places. I trawl Amazon's free (some can be as much as £.77) books and find some real gems in there from time to time.
The other thing is that you should get the free ebook library software,
Calibre. It not only manages your eBook library (and does it well), but it'll fetch RSS feeds from websites and magazines and newspapers and send them to your reader in eBook format, which is quite cool - you can end up reading something like the New York Times (or any major newspaper or magazine) on your eReader, and it'll even put a Table of Contents together. Really cool stuff out there for eReaders!