Depends where you want to get your books more than anything else.
If you don't mind paying market price for the books you want to read, and if you want to read the latest best sellers then your going to want to look at either Kindle or Sony.
If you don't mind reading Public Domain classics, or your a fair pirate, or if you want more control over your ereader, then you might want to look at the Ectaco Jetbook or Jetbook Lite. Jetbook uses a Li-ion battery (like a big cell phone battery) gets up to 20 hours on a charge, has an SD card slot with support for up to 32 gig sd or sdhc cards. Jetbook lite runs on 4 AA battery's (think rechargeable) and gets 20 hours or more from a set.
Jetbook Mini is just about to debut, it runs for longer on AAA batterys, but currently only supports 2 file formats. Jetbook and Jetbook lite support about 10.
Jetbook has no wifi or 3g, but your not paying for it either.
It does come with a USB cable, but a USB to SD card adapter costs about 3$ and is considerably faster.
Jetbook's are currently running around 100$ at newegg, Jetbook Lite has been seen as low as 70$ at newegg.
Last, check out Mobileread
http://www.mobileread.com/forums/index.phpJust about everything having to do with ebooks and ereaders can be found here, along with a lot of great books.
FYI in the past year I've bought 4 Jetbooks, and 3 lites. Wife uses 1, daughter is using my old one, my Dad and my father in law each got one, I'm currently reading on a JBL, we have one JBL going for my wifes boss for christmas, and I have a JBL new in the box as a spare.

FYI a 2 gig SD card will hold enough books that it will take most people a decade to even come close to reading them all. I have over 4000 books on my 4gig card, and its 2/3rds empty.
If you want to talk about sources I'd suggest a PM.
